feat: rich markdown renderer with @mention badges and autocomplete

- Add MarkdownRenderer with syntax-highlighted code blocks (prism-react-renderer),
  GFM tables, and Discord-style theming
- Add MentionBadge that resolves user IDs to display names with role colors
- Add MentionPopover autocomplete triggered by @ in MessageInput
- Fix mention:// URL sanitization — allowlist the scheme in urlTransform so
  react-markdown v9 passes it through to the component override
- Highlight messages that mention the current user (amber border)
- Bump rate limit from 60 to 200 req/min

packages/web/package.json

@@ -13,10 +13,12 @@
     "@opencord/shared": "workspace:*",
     "@sapphi-red/web-noise-suppressor": "^0.3.5",
     "livekit-client": "^2.9.0",
+    "prism-react-renderer": "^2.4.1",
     "react": "^18.3.1",
     "react-dom": "^18.3.1",
     "react-markdown": "^9.0.1",
     "react-router-dom": "^6.28.0",
+    "remark-gfm": "^4.0.1",
     "zustand": "^5.0.2"
   },
   "devDependencies": {