feat: full Huf app PWA

[esafwan]

Summary

Adds full Huf app PWA infrastructure for /huf instead of a separate chat-only PWA shell.

Changes

• Configures the Vite PWA manifest as the full Huf app:
  • name: Huf
  • short_name: Huf
  • start_url: /huf
  • scope: /huf/
  • display: standalone
• Builds frontend assets under /assets/huf/frontend/ via huf/public/frontend.
• Copies the generated app shell to huf/www/huf.html.
• Copies generated PWA route files to huf/www/huf/:
  • /huf/sw.js
  • /huf/manifest.webmanifest
  • /huf/workbox-*.js
• Registers the service worker from /huf/sw.js with scope /huf/ so it can control the installed app path.
• Keeps service worker caching conservative:
  • static frontend assets use StaleWhileRevalidate
  • /api/* uses NetworkOnly
  • /huf/stream/* uses NetworkOnly
• Removes the duplicated standalone chat-only page and /ui/chat route ownership from this branch.

Branch split

• This branch owns shared full-app PWA infrastructure for /huf.
• feat_chat_ui owns the standalone chat-only UI and the route logic that sends chat-only users into /huf/ui/chat.

Validation

• yarn typecheck passed in frontend/.
• yarn build passed in frontend/.
• Verified generated manifest values for name, short_name, start_url, scope, and display.

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