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/object.assign/shim.js

@@ -0,0 +1,14 @@
+'use strict';
+
+var define = require('define-properties');
+var getPolyfill = require('./polyfill');
+
+module.exports = function shimAssign() {
+	var polyfill = getPolyfill();
+	define(
+		Object,
+		{ assign: polyfill },
+		{ assign: function () { return Object.assign !== polyfill; } }
+	);
+	return polyfill;
+};