fix(comments): unify rendering with RichTextEditor, fix mention/link colors

- Comment display: replace <Markdown> with <RichTextEditor editable={false}>
- Link color: primary → brand (blue)
- Mention color: brand → primary + semibold
- Add MentionHoverCard component with HoverCardTrigger render={<span />}
- Markdown.tsx: sync mention style to text-primary font-semibold

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

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

@@ -126,7 +126,7 @@
 
 /* Links */
 .rich-text-editor a {
-  color: var(--primary);
+  color: var(--brand);
   text-decoration: none;
 }
 
@@ -137,8 +137,8 @@
 
 /* Mentions */
 .rich-text-editor .mention {
-  color: var(--brand);
-  font-weight: 500;
+  color: var(--primary);
+  font-weight: 600;
   text-decoration: none;
   margin: 0 0.125rem;
 }