feat: add PWA manifest and service worker

Add Progressive Web App support for installable mobile experience
on iOS Safari and Android Chrome.

- static/manifest.json: app name, standalone display, dark theme,
  icon references for 192px and 512px
- static/sw.js: service worker with cache-first strategy for static
  assets, network-first for HTML/HTMX, offline fallback to cache
- static/icon-192.png, static/icon-512.png: placeholder app icons
- Apple meta tags: apple-mobile-web-app-capable, status bar style,
  apple-touch-icon for iOS Add to Home Screen
- Service worker registration in base layout template

Closes leeworks-agents/gitea-mobile#6

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

static/manifest.json

@@ -0,0 +1,24 @@
+{
+  "name": "Gitea Mobile",
+  "short_name": "Gitea",
+  "description": "Mobile-first PWA for managing Gitea issues and pull requests",
+  "start_url": "/",
+  "display": "standalone",
+  "background_color": "#0d1117",
+  "theme_color": "#161b22",
+  "orientation": "portrait",
+  "icons": [
+    {
+      "src": "/static/icon-192.png",
+      "sizes": "192x192",
+      "type": "image/png",
+      "purpose": "any maskable"
+    },
+    {
+      "src": "/static/icon-512.png",
+      "sizes": "512x512",
+      "type": "image/png",
+      "purpose": "any maskable"
+    }
+  ]
+}