feat: standalone chat-only UI

[esafwan]

Summary

Adds the standalone chat-only UI branch and route behavior for users who should only access chat.

Changes

• Adds the standalone chat-only route ownership:
  • /huf/ui/chat
  • /huf/ui/chat/:chatId
• Adds ChatOnlyPage and components/chat-only/* for the mobile-first chat-only experience.
• Adds chat-only header/layout behavior with logo, agent selector, conversation drawer, and logout entry point.
• Adds route-level permission behavior for chat-only users.

Access behavior

A chat-only user is treated as a user with chat.use and no other app capability.

• Chat-only users opening /huf, /huf/chat, or full-app routes are redirected to /huf/ui/chat.
• Chat-only users can access /huf/ui/chat, /huf/ui/chat/:chatId, and /huf/view/:messageId.
• Full-app users stay on the normal Huf routes.
• /huf/chat remains the desktop/full-app chat route.

Branch split

• This branch owns the chat-only UI and permission-based route behavior.
• feat_pwa owns shared full-app PWA infrastructure for /huf; the installed PWA opens /huf, then this branch's route logic can redirect chat-only users into /huf/ui/chat.

Validation

• yarn typecheck passed in frontend/.

Bench validation was not run in this worktree because it is not a bench environment.