refactor(editor): migrate to @tiptap/markdown, fix mention rendering

- Replace tiptap-markdown with official @tiptap/markdown (markdown→JSON direct, skip DOM)
- Add contentType:"markdown" for proper \n\n paragraph parsing
- Fix mention renderHTML: use mergeAttributes for class/data-type, <a>→<span>
- Fix type attribute leak: add renderHTML:()=>({}) to suppress raw "type" attr
- Link style: permanent underline → hover-only underline (matches read-only)
- Mention style: primary+background pill → brand color text only
- Comment edit: replace <input> with RichTextEditor for consistency

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

apps/web/components/common/rich-text-editor.css

@@ -127,18 +127,20 @@
 /* Links */
 .rich-text-editor a {
   color: var(--primary);
+  text-decoration: none;
+}
+
+.rich-text-editor a:hover {
   text-decoration: underline;
   text-underline-offset: 2px;
 }
 
 /* Mentions */
 .rich-text-editor .mention {
-  color: var(--primary);
-  background: color-mix(in srgb, var(--primary) 8%, transparent);
-  padding: 0 0.2em;
-  border-radius: calc(var(--radius) * 0.5);
+  color: var(--brand);
   font-weight: 500;
   text-decoration: none;
+  margin: 0 0.125rem;
 }
 
 /* Strong / emphasis */