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/issues/components/comment-input.tsx

@@ -3,7 +3,7 @@
 import { useRef, useState } from "react";
 import { ArrowUp, Loader2 } from "lucide-react";
 import { Button } from "@/components/ui/button";
-import { RichTextEditor, type RichTextEditorRef } from "@/components/common/rich-text-editor";
+import { ContentEditor, type ContentEditorRef } from "@/features/editor";
 import { FileUploadButton } from "@/components/common/file-upload-button";
 import { useFileUpload } from "@/shared/hooks/use-file-upload";
 
@@ -13,7 +13,7 @@ interface CommentInputProps {
 }
 
 function CommentInput({ issueId, onSubmit }: CommentInputProps) {
-  const editorRef = useRef<RichTextEditorRef>(null);
+  const editorRef = useRef<ContentEditorRef>(null);
   const [isEmpty, setIsEmpty] = useState(true);
   const [submitting, setSubmitting] = useState(false);
   const { uploadWithToast } = useFileUpload();
@@ -38,7 +38,7 @@ function CommentInput({ issueId, onSubmit }: CommentInputProps) {
   return (
     <div className="relative flex max-h-56 flex-col rounded-lg bg-card pb-8 ring-1 ring-border">
       <div className="flex-1 min-h-0 overflow-y-auto px-3 py-2">
-        <RichTextEditor
+        <ContentEditor
           ref={editorRef}
           placeholder="Leave a comment..."
           onUpdate={(md) => setIsEmpty(!md.trim())}