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/package.json

@@ -0,0 +1,27 @@
+{
+  "name": "tilfluktsrom-pwa",
+  "version": "1.0.0",
+  "description": "Norwegian Emergency Shelter Finder - PWA",
+  "type": "module",
+  "scripts": {
+    "dev": "vite",
+    "build": "tsc && vite build",
+    "preview": "vite preview",
+    "fetch-shelters": "tsx scripts/fetch-shelters.ts",
+    "test": "vitest run",
+    "test:watch": "vitest"
+  },
+  "devDependencies": {
+    "@types/leaflet": "^1.9.8",
+    "typescript": "^5.4.0",
+    "vite": "^5.4.0",
+    "vite-plugin-pwa": "^0.20.0",
+    "vitest": "^2.0.0",
+    "tsx": "^4.16.0"
+  },
+  "dependencies": {
+    "idb": "^8.0.0",
+    "leaflet": "^1.9.4",
+    "leaflet.offline": "^3.2.0"
+  }
+}