refactor(web): improve locale/theme preference initialization

- Extract preference logic into dedicated hooks (useUserLocale, useUserTheme)
- Add applyLocaleEarly() for consistent early application
- Remove applyUserPreferences() from user store (now redundant)
- Simplify App.tsx by moving effects to custom hooks
- Make locale/theme handling consistent and reactive
- Clean up manual preference calls from sign-in flows

Fixes locale not overriding localStorage on user login.
Improves maintainability with better separation of concerns.

web/src/App.tsx

 import { observer } from "mobx-react-lite";
 import { useEffect } from "react";
-import { useTranslation } from "react-i18next";
 import { Outlet } from "react-router-dom";
 import useNavigateTo from "./hooks/useNavigateTo";
-import { instanceStore, userStore } from "./store";
+import { useUserLocale } from "./hooks/useUserLocale";
+import { useUserTheme } from "./hooks/useUserTheme";
+import { instanceStore } from "./store";
 import { cleanupExpiredOAuthState } from "./utils/oauth";
-import { getThemeWithFallback, loadTheme, setupSystemThemeListener } from "./utils/theme";
 
 const App = observer(() => {
-  const { i18n } = useTranslation();
   const navigateTo = useNavigateTo();
   const instanceProfile = instanceStore.state.profile;
-  const userGeneralSetting = userStore.state.userGeneralSetting;
   const instanceGeneralSetting = instanceStore.state.generalSetting;
 
+  // Apply user preferences reactively
+  useUserLocale();
+  useUserTheme();
+
   // Clean up expired OAuth states on app initialization
   useEffect(() => {
     cleanupExpiredOAuthState();
@@ -54,45 +56,6 @@ const App = observer(() => {
     link.href = instanceGeneralSetting.customProfile.logoUrl || "/logo.webp";
   }, [instanceGeneralSetting.customProfile]);
 
-  // Update HTML lang and dir attributes based on current locale
-  useEffect(() => {
-    const currentLocale = i18n.language;
-    document.documentElement.setAttribute("lang", currentLocale);
-    if (["ar", "fa"].includes(currentLocale)) {
-      document.documentElement.setAttribute("dir", "rtl");
-    } else {
-      document.documentElement.setAttribute("dir", "ltr");
-    }
-  }, [i18n.language]);
-
-  // Apply theme when user setting changes
-  useEffect(() => {
-    if (!userGeneralSetting) {
-      return;
-    }
-    const theme = getThemeWithFallback(userGeneralSetting.theme);
-    loadTheme(theme);
-  }, [userGeneralSetting?.theme]);
-
-  // Listen for system theme changes when using "system" theme
-  useEffect(() => {
-    const theme = getThemeWithFallback(userGeneralSetting?.theme);
-
-    // Only set up listener if theme is "system"
-    if (theme !== "system") {
-      return;
-    }
-
-    // Set up listener for OS theme preference changes
-    const cleanup = setupSystemThemeListener(() => {
-      // Reload theme when system preference changes
-      loadTheme(theme);
-    });
-
-    // Cleanup listener on unmount or when theme changes
-    return cleanup;
-  }, [userGeneralSetting?.theme]);
-
   return <Outlet />;
 });
 

web/src/components/Settings/PreferencesSection.tsx

 import { observer } from "mobx-react-lite";
 import { Select, SelectContent, SelectItem, SelectTrigger, SelectValue } from "@/components/ui/select";
-import i18n from "@/i18n";
 import { userStore } from "@/store";
 import { Visibility } from "@/types/proto/api/v1/memo_service";
 import { UserSetting_GeneralSetting } from "@/types/proto/api/v1/user_service";
-import { useTranslate } from "@/utils/i18n";
+import { loadLocale, useTranslate } from "@/utils/i18n";
 import { convertVisibilityFromString, convertVisibilityToString } from "@/utils/memo";
 import { loadTheme } from "@/utils/theme";
 import LocaleSelect from "../LocaleSelect";
@@ -20,8 +19,8 @@ const PreferencesSection = observer(() => {
   const generalSetting = userStore.state.userGeneralSetting;
 
   const handleLocaleSelectChange = async (locale: Locale) => {
-    // Apply locale immediately for instant UI feedback
-    i18n.changeLanguage(locale);
+    // Apply locale immediately for instant UI feedback and persist to localStorage
+    loadLocale(locale);
     // Persist to user settings
     await userStore.updateUserGeneralSetting({ locale }, ["locale"]);
   };

web/src/hooks/index.ts

@@ -6,3 +6,5 @@ export * from "./useMemoFilters";
 export * from "./useMemoSorting";
 export * from "./useNavigateTo";
 export * from "./useResponsiveWidth";
+export * from "./useUserLocale";
+export * from "./useUserTheme";

web/src/hooks/useUserLocale.ts

+import { useEffect } from "react";
+import { useTranslation } from "react-i18next";
+import { userStore } from "@/store";
+import { getLocaleWithFallback, loadLocale } from "@/utils/i18n";
+
+/**
+ * Hook that reactively applies user locale preference.
+ * Priority: User setting → localStorage → browser language
+ */
+export const useUserLocale = () => {
+  const { i18n } = useTranslation();
+  const userGeneralSetting = userStore.state.userGeneralSetting;
+
+  // Apply locale when user setting changes or user logs in
+  useEffect(() => {
+    if (!userGeneralSetting) {
+      return;
+    }
+    const locale = getLocaleWithFallback(userGeneralSetting.locale);
+    loadLocale(locale);
+  }, [userGeneralSetting?.locale]);
+
+  // Update HTML lang and dir attributes based on current locale
+  useEffect(() => {
+    const currentLocale = i18n.language;
+    document.documentElement.setAttribute("lang", currentLocale);
+
+    // RTL languages
+    if (["ar", "fa"].includes(currentLocale)) {
+      document.documentElement.setAttribute("dir", "rtl");
+    } else {
+      document.documentElement.setAttribute("dir", "ltr");
+    }
+  }, [i18n.language]);
+};

web/src/hooks/useUserTheme.ts

+import { useEffect } from "react";
+import { userStore } from "@/store";
+import { getThemeWithFallback, loadTheme, setupSystemThemeListener } from "@/utils/theme";
+
+/**
+ * Hook that reactively applies user theme preference.
+ * Priority: User setting → localStorage → system preference
+ */
+export const useUserTheme = () => {
+  const userGeneralSetting = userStore.state.userGeneralSetting;
+
+  // Apply theme when user setting changes or user logs in
+  useEffect(() => {
+    if (!userGeneralSetting) {
+      return;
+    }
+    const theme = getThemeWithFallback(userGeneralSetting.theme);
+    loadTheme(theme);
+  }, [userGeneralSetting?.theme]);
+
+  // Listen for system theme changes when using "system" theme
+  useEffect(() => {
+    const theme = getThemeWithFallback(userGeneralSetting?.theme);
+
+    // Only set up listener if theme is "system"
+    if (theme !== "system") {
+      return;
+    }
+
+    // Set up listener for OS theme preference changes
+    const cleanup = setupSystemThemeListener(() => {
+      loadTheme(theme);
+    });
+
+    return cleanup;
+  }, [userGeneralSetting?.theme]);
+};

web/src/main.tsx

@@ -9,13 +9,15 @@ import router from "./router";
 // Configure MobX before importing any stores
 import "./store/config";
 import { initialInstanceStore } from "./store/instance";
-import userStore, { initialUserStore } from "./store/user";
+import { initialUserStore } from "./store/user";
+import { applyLocaleEarly } from "./utils/i18n";
 import { applyThemeEarly } from "./utils/theme";
 import "leaflet/dist/leaflet.css";
 
-// Apply theme early to prevent flash of wrong theme
+// Apply theme and locale early to prevent flash of wrong theme/language
 // This uses localStorage as the source before user settings are loaded
 applyThemeEarly();
+applyLocaleEarly();
 
 const Main = observer(() => (
   <>
@@ -29,10 +31,6 @@ const Main = observer(() => (
   await initialInstanceStore();
   await initialUserStore();
 
-  // Apply user preferences (theme & locale) after user settings are loaded
-  // This will override the early theme with user's actual preference
-  userStore.applyUserPreferences();
-
   const container = document.getElementById("root");
   const root = createRoot(container as HTMLElement);
   root.render(<Main />);

web/src/store/user.ts

 import { uniqueId } from "lodash-es";
 import { computed, makeAutoObservable } from "mobx";
 import { authServiceClient, shortcutServiceClient, userServiceClient } from "@/grpcweb";
-import i18n from "@/i18n";
 import { Shortcut } from "@/types/proto/api/v1/shortcut_service";
 import {
   User,
@@ -14,8 +13,6 @@ import {
   UserSetting_WebhooksSetting,
   UserStats,
 } from "@/types/proto/api/v1/user_service";
-import { getLocaleWithFallback } from "@/utils/i18n";
-import { getThemeWithFallback, loadTheme } from "@/utils/theme";
 import { createRequestKey, RequestDeduplicator, StoreError } from "./store-utils";
 
 class LocalState {
@@ -284,20 +281,6 @@ const userStore = (() => {
     state.statsStateId = id;
   };
 
-  // Applies user preferences (theme and locale) with proper fallbacks
-  // This should be called after user settings are loaded
-  const applyUserPreferences = () => {
-    const generalSetting = state.userGeneralSetting;
-
-    // Apply theme with fallback: user setting -> localStorage -> system
-    const theme = getThemeWithFallback(generalSetting?.theme);
-    loadTheme(theme);
-
-    // Apply locale with fallback: user setting -> browser language
-    const locale = getLocaleWithFallback(generalSetting?.locale);
-    i18n.changeLanguage(locale);
-  };
-
   return {
     state,
     getOrFetchUserByName,
@@ -314,7 +297,6 @@ const userStore = (() => {
     deleteNotification,
     fetchUserStats,
     setStatsStateId,
-    applyUserPreferences,
   };
 })();
 

web/src/utils/i18n.ts

@@ -3,6 +3,25 @@ import { useTranslation } from "react-i18next";
 import i18n, { locales, TLocale } from "@/i18n";
 import enTranslation from "@/locales/en.json";
 
+const LOCALE_STORAGE_KEY = "memos-locale";
+
+const getStoredLocale = (): Locale | null => {
+  try {
+    const stored = localStorage.getItem(LOCALE_STORAGE_KEY);
+    return stored && locales.includes(stored) ? (stored as Locale) : null;
+  } catch {
+    return null;
+  }
+};
+
+const setStoredLocale = (locale: Locale): void => {
+  try {
+    localStorage.setItem(LOCALE_STORAGE_KEY, locale);
+  } catch {
+    // localStorage might not be available
+  }
+};
+
 export const findNearestMatchedLanguage = (language: string): Locale => {
   if (locales.includes(language as TLocale)) {
     return language as Locale;
@@ -54,17 +73,42 @@ export const isValidateLocale = (locale: string | undefined | null): boolean =>
 
 // Gets the locale to use with proper priority:
 // 1. User setting (if logged in and has preference)
-// 2. Browser language preference
+// 2. localStorage (from previous session)
+// 3. Browser language preference
 export const getLocaleWithFallback = (userLocale?: string): Locale => {
   // Priority 1: User setting (if logged in and valid)
   if (userLocale && isValidateLocale(userLocale)) {
     return userLocale as Locale;
   }
 
-  // Priority 2: Browser language
+  // Priority 2: localStorage
+  const stored = getStoredLocale();
+  if (stored) {
+    return stored;
+  }
+
+  // Priority 3: Browser language
   return findNearestMatchedLanguage(navigator.language);
 };
 
+// Applies and persists a locale setting
+export const loadLocale = (locale: string): Locale => {
+  const validLocale = isValidateLocale(locale) ? (locale as Locale) : findNearestMatchedLanguage(navigator.language);
+  setStoredLocale(validLocale);
+  i18n.changeLanguage(validLocale);
+  return validLocale;
+};
+
+/**
+ * Applies locale early during initial page load to prevent language flash.
+ * Uses only localStorage and browser language (no user settings yet).
+ */
+export const applyLocaleEarly = (): void => {
+  const stored = getStoredLocale();
+  const locale = stored ?? findNearestMatchedLanguage(navigator.language);
+  loadLocale(locale);
+};
+
 // Get the display name for a locale in its native language
 export const getLocaleDisplayName = (locale: string): string => {
   try {