Add PWA support and mobile responsive layout

- PWA manifest, service worker (stale-while-revalidate for static assets,
  network-only for API), and app icons for installability
- Root-scoped /manifest.json and /sw.js routes in FastAPI
- New mobile.css with responsive breakpoints at 768/600/400px:
  fixed bottom tab bar on phones, single-column cards, full-screen modals,
  compact header toolbar, touch-friendly targets
- Fix modal-content-wide min-width overflow on small screens
- Update README with Camera, OpenRGB, and PWA features

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

TODO.md

@@ -58,6 +58,4 @@ Priority: `P1` quick win · `P2` moderate · `P3` large effort
 - [ ] `P2` **Tags / groups for cards** — Assign tags to devices, targets, and sources; filter and group cards by tag
   - Complexity: medium — new `tags: List[str]` field on all card entities; tag CRUD API; filter bar UI per section; tag badge rendering on cards; persistence migration
   - Impact: medium-high — essential for setups with many devices/targets; enables quick filtering (e.g. "bedroom", "desk", "gaming")
-- [ ] `P3` **PWA / mobile layout** — Mobile-first layout + "Add to Home Screen" manifest
-  - Complexity: medium-large — responsive CSS overhaul for all tabs; service worker for offline caching; manifest.json; touch-friendly controls (larger tap targets, swipe gestures)
-  - Impact: high — phone control is a top user request; current desktop-first layout is unusable on mobile
+- [x] `P3` **PWA / mobile layout** — Mobile-first layout + "Add to Home Screen" manifest