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/extensions/markdown-paste.ts

@@ -0,0 +1,31 @@
+import { Extension } from "@tiptap/core";
+import { Plugin, PluginKey } from "@tiptap/pm/state";
+import { Slice } from "@tiptap/pm/model";
+
+export function createMarkdownPasteExtension() {
+  return Extension.create({
+    name: "markdownPaste",
+    addProseMirrorPlugins() {
+      const { editor } = this;
+      return [
+        new Plugin({
+          key: new PluginKey("markdownPaste"),
+          props: {
+            clipboardTextParser(text, _context, plainText) {
+              if (!plainText && editor.markdown) {
+                const json = editor.markdown.parse(text);
+                const node = editor.schema.nodeFromJSON(json);
+                return Slice.maxOpen(node.content);
+              }
+              // Plain text fallback
+              const p = editor.schema.nodes.paragraph!;
+              const doc = editor.schema.nodes.doc!;
+              const paragraph = p.create(null, text ? editor.schema.text(text) : undefined);
+              return new Slice(doc.create(null, paragraph).content, 0, 0);
+            },
+          },
+        }),
+      ];
+    },
+  });
+}