feat(web): add Mermaid diagram support in markdown (#5242)

Co-authored-by: Claude <noreply@anthropic.com>

web/src/components/MemoContent/CodeBlock.tsx

 import { CheckIcon, CopyIcon } from "lucide-react";
 import { useState } from "react";
 import { cn } from "@/lib/utils";
+import { MermaidBlock } from "./MermaidBlock";
 
 interface PreProps {
   children?: React.ReactNode;
@@ -19,6 +20,15 @@ export const CodeBlock = ({ children, className, ...props }: PreProps) => {
   const match = /language-(\w+)/.exec(codeClassName);
   const language = match ? match[1] : "";
 
+  // If it's a mermaid block, render with MermaidBlock component
+  if (language === "mermaid") {
+    return (
+      <MermaidBlock className={className} {...props}>
+        {children}
+      </MermaidBlock>
+    );
+  }
+
   const handleCopy = async () => {
     try {
       await navigator.clipboard.writeText(codeContent);

web/src/components/MemoContent/MermaidBlock.tsx

+import mermaid from "mermaid";
+import { observer } from "mobx-react-lite";
+import { useEffect, useMemo, useRef, useState } from "react";
+import { cn } from "@/lib/utils";
+import { instanceStore, userStore } from "@/store";
+import { resolveTheme } from "@/utils/theme";
+
+interface MermaidBlockProps {
+  children?: React.ReactNode;
+  className?: string;
+}
+
+/**
+ * Maps app theme to Mermaid theme
+ * @param appTheme - The resolved app theme
+ * @returns Mermaid theme name
+ */
+const getMermaidTheme = (appTheme: string): "default" | "dark" => {
+  switch (appTheme) {
+    case "default-dark":
+      return "dark";
+    case "default":
+    case "paper":
+    case "whitewall":
+    default:
+      return "default";
+  }
+};
+
+export const MermaidBlock = observer(({ children, className }: MermaidBlockProps) => {
+  const containerRef = useRef<HTMLDivElement>(null);
+  const [svg, setSvg] = useState<string>("");
+  const [error, setError] = useState<string>("");
+
+  // Extract the code element and its content
+  const codeElement = children as React.ReactElement;
+  const codeContent = String(codeElement?.props?.children || "").replace(/\n$/, "");
+
+  // Get current theme from store (reactive via MobX observer)
+  // This will automatically trigger re-render when theme changes
+  const currentTheme = useMemo(() => {
+    const userTheme = userStore.state.userGeneralSetting?.theme;
+    const instanceTheme = instanceStore.state.theme;
+    const theme = userTheme || instanceTheme;
+    return resolveTheme(theme);
+  }, [userStore.state.userGeneralSetting?.theme, instanceStore.state.theme]);
+
+  // Render diagram when content or theme changes
+  useEffect(() => {
+    const renderDiagram = async () => {
+      if (!codeContent || !containerRef.current) {
+        return;
+      }
+
+      try {
+        // Generate a unique ID for this diagram
+        const id = `mermaid-${Math.random().toString(36).substring(7)}`;
+
+        // Get the appropriate Mermaid theme for current app theme
+        const mermaidTheme = getMermaidTheme(currentTheme);
+
+        // Initialize mermaid with current theme
+        mermaid.initialize({
+          startOnLoad: false,
+          theme: mermaidTheme,
+          securityLevel: "strict",
+          fontFamily: "inherit",
+        });
+
+        // Render the mermaid diagram
+        const { svg: renderedSvg } = await mermaid.render(id, codeContent);
+        setSvg(renderedSvg);
+        setError("");
+      } catch (err) {
+        console.error("Failed to render mermaid diagram:", err);
+        setError(err instanceof Error ? err.message : "Failed to render diagram");
+      }
+    };
+
+    renderDiagram();
+  }, [codeContent, currentTheme]);
+
+  // If there's an error, fall back to showing the code
+  if (error) {
+    return (
+      <div className="w-full">
+        <div className="text-sm text-destructive mb-2">Mermaid Error: {error}</div>
+        <pre className={className}>
+          <code className="language-mermaid">{codeContent}</code>
+        </pre>
+      </div>
+    );
+  }
+
+  return (
+    <div
+      ref={containerRef}
+      className={cn("mermaid-diagram w-full flex justify-center items-center my-4 overflow-x-auto", className)}
+      dangerouslySetInnerHTML={{ __html: svg }}
+    />
+  );
+});