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>

server/src/wled_controller/static/css/modal.css

@@ -545,13 +545,18 @@
 
 .modal-content-wide {
     width: fit-content;
-    min-width: 500px;
     max-width: calc(100vw - 40px);
     max-height: calc(100vh - 40px);
     display: flex;
     flex-direction: column;
 }
 
+@media (min-width: 769px) {
+    .modal-content-wide {
+        min-width: 500px;
+    }
+}
+
 .modal-content-wide .modal-body {
     overflow-y: auto;
     scrollbar-gutter: stable;