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/is-shared-array-buffer/index.js

@@ -0,0 +1,24 @@
+'use strict';
+
+var callBound = require('call-bound');
+
+/** @type {undefined | ((thisArg: SharedArrayBuffer) => number)} */
+var $byteLength = callBound('SharedArrayBuffer.prototype.byteLength', true);
+
+/** @type {import('.')} */
+module.exports = $byteLength
+	? function isSharedArrayBuffer(obj) {
+		if (!obj || typeof obj !== 'object') {
+			return false;
+		}
+		try {
+			// @ts-expect-error TS can't figure out this closed-over variable is non-nullable, and it's fine that `obj` might not be a SAB
+			$byteLength(obj);
+			return true;
+		} catch (e) {
+			return false;
+		}
+	}
+	: function isSharedArrayBuffer(_obj) { // eslint-disable-line no-unused-vars
+		return false;
+	};