#9373: UI: add PWA support to control UI

## Summary - add PWA manifest and icons - register service worker for offline shell + asset caching - add PWA meta tags ## Testing - pnpm ui:build (warns: Node v21.7.3 < 22.12.0) <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> This PR adds Progressive Web App (PWA) support to the UI by introducing a web manifest + PWA icons, adding PWA-related meta tags to `ui/index.html`, and registering a new service worker (`ui/public/sw.js`) from app startup (`ui/src/main.ts` -> `ui/src/pwa.ts`). The service worker implements an install-time core-asset precache and runtime caching for navigations and static assets to enable an offline shell. <h3>Confidence Score: 3/5</h3> - Mostly safe to merge, but PWA registration/caching will break under non-root base paths. - Changes are localized and straightforward, but the current service worker registration URL and precache pathing are inconsistent with typical Vite public asset serving and cache matching semantics; this will cause the PWA/offline behavior to fail in common deployment configurations. - ui/src/pwa.ts, ui/public/sw.js <!-- greptile_other_comments_section --> _{(2/5) Greptile learns from your feedback when you react with thumbs up/down!} <!-- /greptile_comment -->