Add progressive web app companion for cross-platform access

Vite + TypeScript PWA that mirrors the Android app's core features:
- Pre-processed shelter data (build-time UTM33N→WGS84 conversion)
- Leaflet map with shelter markers, user location, and offline tiles
- Canvas compass arrow (ported from DirectionArrowView.kt)
- IndexedDB shelter cache with 7-day staleness check
- Service worker with CacheFirst tiles and precached app shell
- i18n for en, nb, nn (ported from Android strings.xml)
- iOS/Android compass handling with low-pass filter
- Respects user map interaction (no auto-snap on pan/zoom)
- Build revision cache-breaker for reliable SW updates

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

pwa/vite.config.ts

@@ -0,0 +1,56 @@
+import { defineConfig } from 'vite';
+import { VitePWA } from 'vite-plugin-pwa';
+
+export default defineConfig({
+  define: {
+    // Injected as a global — changes every build, breaking any stale cache
+    __BUILD_REVISION__: JSON.stringify(
+      `${new Date().toISOString().slice(0, 16)}`,
+    ),
+  },
+  plugins: [
+    VitePWA({
+      registerType: 'autoUpdate',
+      workbox: {
+        // Precache all built assets (JS/CSS get content hashes from Vite;
+        // files in public/ like shelters.json get a Workbox revision hash
+        // computed from their content, so any change triggers re-fetch).
+        globPatterns: ['**/*.{js,css,html,json,png,svg,ico,webmanifest}'],
+
+        // Remove old precache entries from previous builds
+        cleanupOutdatedCaches: true,
+
+        // SPA: serve index.html for all navigation requests
+        navigateFallback: '/index.html',
+
+        // Vite already hashes JS/CSS filenames — skip Workbox's
+        // cache-bust query parameter for those
+        dontCacheBustURLsMatching: /\.[0-9a-f]{8}\./,
+
+        // Runtime caching for map tiles (not precached — cached as viewed)
+        runtimeCaching: [
+          {
+            urlPattern: /^https:\/\/[abc]\.tile\.openstreetmap\.org\/.*/,
+            handler: 'CacheFirst',
+            options: {
+              cacheName: 'osm-tiles',
+              expiration: {
+                maxEntries: 5000,
+                maxAgeSeconds: 30 * 24 * 60 * 60, // 30 days
+              },
+              cacheableResponse: {
+                statuses: [0, 200],
+              },
+            },
+          },
+        ],
+      },
+      manifest: false, // We provide our own manifest.webmanifest
+    }),
+  ],
+  resolve: {
+    alias: {
+      '@': '/src',
+    },
+  },
+});