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"
+    }
+  ]
+}

static/sw.js

@@ -0,0 +1,86 @@
+// Service Worker for Gitea Mobile PWA
+// Caches the app shell for offline/fast loading.
+
+const CACHE_NAME = 'gitea-mobile-v1';
+const APP_SHELL = [
+  '/',
+  '/static/style.css',
+  '/static/manifest.json',
+  '/static/icon-192.png',
+  '/static/icon-512.png',
+  'https://unpkg.com/htmx.org@1.9.10'
+];
+
+// Install: cache app shell resources.
+self.addEventListener('install', (event) => {
+  event.waitUntil(
+    caches.open(CACHE_NAME).then((cache) => {
+      return cache.addAll(APP_SHELL);
+    })
+  );
+  // Activate immediately.
+  self.skipWaiting();
+});
+
+// Activate: clean up old caches.
+self.addEventListener('activate', (event) => {
+  event.waitUntil(
+    caches.keys().then((cacheNames) => {
+      return Promise.all(
+        cacheNames
+          .filter((name) => name !== CACHE_NAME)
+          .map((name) => caches.delete(name))
+      );
+    })
+  );
+  // Take control of all clients immediately.
+  self.clients.claim();
+});
+
+// Fetch: network-first for API/HTML, cache-first for static assets.
+self.addEventListener('fetch', (event) => {
+  const url = new URL(event.request.url);
+
+  // Cache-first for static assets.
+  if (url.pathname.startsWith('/static/')) {
+    event.respondWith(
+      caches.match(event.request).then((cached) => {
+        return cached || fetch(event.request).then((response) => {
+          // Cache the fetched response for next time.
+          const responseClone = response.clone();
+          caches.open(CACHE_NAME).then((cache) => {
+            cache.put(event.request, responseClone);
+          });
+          return response;
+        });
+      })
+    );
+    return;
+  }
+
+  // Network-first for HTML/API requests.
+  if (event.request.headers.get('accept')?.includes('text/html') ||
+      event.request.headers.get('HX-Request')) {
+    event.respondWith(
+      fetch(event.request)
+        .then((response) => {
+          // Cache successful GET responses.
+          if (event.request.method === 'GET' && response.ok) {
+            const responseClone = response.clone();
+            caches.open(CACHE_NAME).then((cache) => {
+              cache.put(event.request, responseClone);
+            });
+          }
+          return response;
+        })
+        .catch(() => {
+          // Fallback to cache if network fails.
+          return caches.match(event.request);
+        })
+    );
+    return;
+  }
+
+  // Default: network only.
+  event.respondWith(fetch(event.request));
+});