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-set/index.js

@@ -0,0 +1,46 @@
+'use strict';
+
+var $Map = typeof Map === 'function' && Map.prototype ? Map : null;
+var $Set = typeof Set === 'function' && Set.prototype ? Set : null;
+
+var exported;
+
+if (!$Set) {
+	/** @type {import('.')} */
+	// eslint-disable-next-line no-unused-vars
+	exported = function isSet(x) {
+		// `Set` is not present in this environment.
+		return false;
+	};
+}
+
+var $mapHas = $Map ? Map.prototype.has : null;
+var $setHas = $Set ? Set.prototype.has : null;
+if (!exported && !$setHas) {
+	/** @type {import('.')} */
+	// eslint-disable-next-line no-unused-vars
+	exported = function isSet(x) {
+		// `Set` does not have a `has` method
+		return false;
+	};
+}
+
+/** @type {import('.')} */
+module.exports = exported || function isSet(x) {
+	if (!x || typeof x !== 'object') {
+		return false;
+	}
+	try {
+		$setHas.call(x);
+		if ($mapHas) {
+			try {
+				$mapHas.call(x);
+			} catch (e) {
+				return true;
+			}
+		}
+		// @ts-expect-error TS can't figure out that $Set is always truthy here
+		return x instanceof $Set; // core-js workaround, pre-v2.5.0
+	} catch (e) {}
+	return false;
+};