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/features/editor/utils/preprocess.ts

@@ -0,0 +1,16 @@
+import { preprocessLinks } from "@/components/markdown/linkify";
+import { preprocessMentionShortcodes } from "@/components/markdown/mentions";
+
+/**
+ * Preprocess a markdown string before loading into Tiptap via contentType: 'markdown'.
+ *
+ * Two string→string transforms:
+ * 1. Legacy mention shortcodes [@ id="..." label="..."] → [@Label](mention://member/id)
+ * 2. Raw URLs → markdown links (so they render as clickable Link nodes)
+ */
+export function preprocessMarkdown(markdown: string): string {
+  if (!markdown) return "";
+  const step1 = preprocessMentionShortcodes(markdown);
+  const step2 = preprocessLinks(step1);
+  return step2;
+}