fix(web): disable setext header syntax (#5314)

Add custom remark plugin to prevent setext headers (headers using === or --- underlines) from being recognized by the markdown parser. The plugin disables the setextUnderline construct at the micromark parser level.

🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

web/src/components/MemoContent/index.tsx

@@ -11,6 +11,7 @@ import useCurrentUser from "@/hooks/useCurrentUser";
 import { cn } from "@/lib/utils";
 import { memoStore } from "@/store";
 import { useTranslate } from "@/utils/i18n";
+import { remarkDisableSetext } from "@/utils/remark-plugins/remark-disable-setext";
 import { remarkPreserveType } from "@/utils/remark-plugins/remark-preserve-type";
 import { remarkTag } from "@/utils/remark-plugins/remark-tag";
 import { isSuperUser } from "@/utils/user";
@@ -59,7 +60,7 @@ const MemoContent = observer((props: MemoContentProps) => {
           onDoubleClick={onDoubleClick}
         >
           <ReactMarkdown
-            remarkPlugins={[remarkGfm, remarkBreaks, remarkMath, remarkTag, remarkPreserveType]}
+            remarkPlugins={[remarkDisableSetext, remarkGfm, remarkBreaks, remarkMath, remarkTag, remarkPreserveType]}
             rehypePlugins={[rehypeRaw, rehypeKatex, [rehypeSanitize, SANITIZE_SCHEMA]]}
             components={{
               // Conditionally render custom components based on AST node type

web/src/utils/remark-plugins/remark-disable-setext.ts

+/**
+ * Remark plugin to disable setext header syntax.
+ *
+ * Setext headers use underlines (=== or ---) to create headings:
+ *   Heading 1
+ *   =========
+ *
+ *   Heading 2
+ *   ---------
+ *
+ * This plugin disables the setext heading construct at the micromark parser level,
+ * preventing these patterns from being recognized as headers.
+ */
+export function remarkDisableSetext(this: any) {
+  const data = this.data();
+
+  add("micromarkExtensions", {
+    disable: {
+      null: ["setextUnderline"],
+    },
+  });
+
+  /**
+   * Add a micromark extension to the parser configuration.
+   */
+  function add(field: string, value: any) {
+    const list = data[field] ? data[field] : (data[field] = []);
+    list.push(value);
+  }
+}