refactor(editor): unify editor into features/editor with single markdown pipeline

Replace three divergent data paths (Marked HTML loading, regex post-processing
saving, separate paste parsing) with one symmetric path through @tiptap/markdown.

Key changes:
- Create features/editor/ module with ContentEditor (unified edit+readonly)
  and TitleEditor, replacing components/common/ editor files
- Load content via contentType: 'markdown' instead of markdownToHtml() hack
- Save content via editor.getMarkdown() directly, no post-processing
- Merge RichTextEditor + ReadonlyEditor into single ContentEditor with
  editable prop
- Extract extensions into separate modules (mention, file-upload,
  markdown-paste, submit-shortcut, code-block-view)
- Extract shared preprocessMentionShortcodes to components/markdown/mentions.ts
- Add copyMarkdown utility for clipboard operations
- Upgrade all @tiptap packages from 3.20.5 to 3.22.1 (lexer isolation fix,
  HTML entity roundtrip fix, table alignment support)
- Delete markdownToHtml.ts, readonly-editor.tsx, and 10 old component files

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

apps/web/components/markdown/Markdown.tsx

@@ -5,6 +5,7 @@ import remarkGfm from 'remark-gfm'
 import { cn } from '@/lib/utils'
 import { CodeBlock, InlineCode } from './CodeBlock'
 import { preprocessLinks } from './linkify'
+import { preprocessMentionShortcodes } from './mentions'
 import { IssueMentionCard } from '@/features/issues/components/issue-mention-card'
 
 /**
@@ -53,27 +54,6 @@ function urlTransform(url: string): string {
   return defaultUrlTransform(url)
 }
 
-/**
- * Convert legacy mention shortcodes [@ id="UUID" label="LABEL"] to markdown
- * link format [@LABEL](mention://member/UUID) so they render as styled mentions.
- */
-function preprocessMentionShortcodes(text: string): string {
-  if (!text.includes('[@ ')) return text
-  return text.replace(
-    /\[@\s+([^\]]*)\]/g,
-    (match, attrString: string) => {
-      const attrs: Record<string, string> = {}
-      const re = /(\w+)="([^"]*)"/g
-      let m
-      while ((m = re.exec(attrString)) !== null) {
-        if (m[1] && m[2] !== undefined) attrs[m[1]] = m[2]
-      }
-      const { id, label } = attrs
-      if (!id || !label) return match
-      return `[@${label}](mention://member/${id})`
-    }
-  )
-}
 
 // File path detection regex - matches paths starting with /, ~/, or ./
 const FILE_PATH_REGEX =

apps/web/components/markdown/index.ts

@@ -2,3 +2,4 @@ export { Markdown, MemoizedMarkdown, type MarkdownProps, type RenderMode } from
 export { CodeBlock, InlineCode, type CodeBlockProps } from './CodeBlock'
 export { StreamingMarkdown, type StreamingMarkdownProps } from './StreamingMarkdown'
 export { preprocessLinks, detectLinks, hasLinks } from './linkify'
+export { preprocessMentionShortcodes } from './mentions'

apps/web/components/markdown/mentions.ts

@@ -0,0 +1,25 @@
+/**
+ * Convert legacy mention shortcodes [@ id="UUID" label="LABEL"] to the
+ * standard markdown link format [@LABEL](mention://member/UUID).
+ *
+ * These shortcodes exist in older database records from a previous mention
+ * serialization format. This function normalises them so downstream parsers
+ * (Tiptap @tiptap/markdown, react-markdown) only need to handle one syntax.
+ */
+export function preprocessMentionShortcodes(text: string): string {
+  if (!text.includes("[@ ")) return text;
+  return text.replace(
+    /\[@\s+([^\]]*)\]/g,
+    (match, attrString: string) => {
+      const attrs: Record<string, string> = {};
+      const re = /(\w+)="([^"]*)"/g;
+      let m;
+      while ((m = re.exec(attrString)) !== null) {
+        if (m[1] && m[2] !== undefined) attrs[m[1]] = m[2];
+      }
+      const { id, label } = attrs;
+      if (!id || !label) return match;
+      return `[@${label}](mention://member/${id})`;
+    },
+  );
+}