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/src/main.ts

@@ -0,0 +1,36 @@
+/**
+ * Entry point: wait for DOM, initialize locale, boot the app.
+ *
+ * The __BUILD_REVISION__ constant is injected by Vite at build time and
+ * changes on every build. Combined with vite-plugin-pwa's autoUpdate
+ * registration, this ensures the service worker detects any new deployment
+ * and swaps in the fresh precache immediately.
+ */
+
+import './styles/main.css';
+import 'leaflet/dist/leaflet.css';
+import { initLocale } from './i18n/i18n';
+import { init } from './app';
+import { setStatus } from './ui/status-bar';
+import { t } from './i18n/i18n';
+
+console.info(`[tilfluktsrom] build ${__BUILD_REVISION__}`);
+
+document.addEventListener('DOMContentLoaded', async () => {
+  initLocale();
+
+  // Request persistent storage (helps prevent iOS eviction)
+  if (navigator.storage?.persist) {
+    await navigator.storage.persist();
+  }
+
+  // Listen for service worker updates — flash a status message when a new
+  // version activates so the user knows they have fresh code/data.
+  if ('serviceWorker' in navigator) {
+    navigator.serviceWorker.addEventListener('controllerchange', () => {
+      setStatus(t('update_success'));
+    });
+  }
+
+  await init();
+});