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/common/code-block-view.tsx

@@ -1,52 +0,0 @@
-"use client";
-
-import { useState } from "react";
-import { NodeViewWrapper, NodeViewContent } from "@tiptap/react";
-import type { NodeViewProps } from "@tiptap/react";
-import { Copy, Check } from "lucide-react";
-
-function CodeBlockView({ node }: NodeViewProps) {
-  const [copied, setCopied] = useState(false);
-  const language = node.attrs.language || "";
-
-  const handleCopy = async () => {
-    const text = node.textContent;
-    if (!text) return;
-    await navigator.clipboard.writeText(text);
-    setCopied(true);
-    setTimeout(() => setCopied(false), 2000);
-  };
-
-  return (
-    <NodeViewWrapper className="code-block-wrapper group/code relative my-2">
-      <div
-        contentEditable={false}
-        className="code-block-header absolute top-0 right-0 z-10 flex items-center gap-1.5 px-2 py-1.5 opacity-0 transition-opacity group-hover/code:opacity-100"
-      >
-        {language && (
-          <span className="text-xs text-muted-foreground select-none">
-            {language}
-          </span>
-        )}
-        <button
-          type="button"
-          onClick={handleCopy}
-          className="flex h-6 w-6 items-center justify-center rounded text-muted-foreground hover:bg-muted hover:text-foreground transition-colors"
-          title="Copy code"
-        >
-          {copied ? (
-            <Check className="h-3.5 w-3.5" />
-          ) : (
-            <Copy className="h-3.5 w-3.5" />
-          )}
-        </button>
-      </div>
-      <pre spellCheck={false}>
-        {/* @ts-expect-error -- NodeViewContent supports as="code" at runtime */}
-        <NodeViewContent as="code" />
-      </pre>
-    </NodeViewWrapper>
-  );
-}
-
-export { CodeBlockView };