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/node_modules/core-js-compat/external.json

@@ -0,0 +1,21 @@
+{
+  "modules": {
+    "android": "61",
+    "bun": "0.1.1",
+    "chrome": "61",
+    "chrome-android": "61",
+    "deno": "1.0",
+    "edge": "16",
+    "firefox": "60",
+    "firefox-android": "60",
+    "ios": "10.3",
+    "node": "13.2",
+    "oculus": "4.0",
+    "opera": "48",
+    "opera-android": "45",
+    "opera_mobile": "45",
+    "quest": "4.0",
+    "safari": "10.1",
+    "samsung": "8.0"
+  }
+}