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/workbox-window/utils/WorkboxEventTarget.js

@@ -0,0 +1,60 @@
+/*
+  Copyright 2019 Google LLC
+
+  Use of this source code is governed by an MIT-style
+  license that can be found in the LICENSE file or at
+  https://opensource.org/licenses/MIT.
+*/
+/**
+ * A minimal `EventTarget` shim.
+ * This is necessary because not all browsers support constructable
+ * `EventTarget`, so using a real `EventTarget` will error.
+ * @private
+ */
+export class WorkboxEventTarget {
+    constructor() {
+        this._eventListenerRegistry = new Map();
+    }
+    /**
+     * @param {string} type
+     * @param {Function} listener
+     * @private
+     */
+    addEventListener(type, listener) {
+        const foo = this._getEventListenersByType(type);
+        foo.add(listener);
+    }
+    /**
+     * @param {string} type
+     * @param {Function} listener
+     * @private
+     */
+    removeEventListener(type, listener) {
+        this._getEventListenersByType(type).delete(listener);
+    }
+    /**
+     * @param {Object} event
+     * @private
+     */
+    dispatchEvent(event) {
+        event.target = this;
+        const listeners = this._getEventListenersByType(event.type);
+        for (const listener of listeners) {
+            listener(event);
+        }
+    }
+    /**
+     * Returns a Set of listeners associated with the passed event type.
+     * If no handlers have been registered, an empty Set is returned.
+     *
+     * @param {string} type The event type.
+     * @return {Set<ListenerCallback>} An array of handler functions.
+     * @private
+     */
+    _getEventListenersByType(type) {
+        if (!this._eventListenerRegistry.has(type)) {
+            this._eventListenerRegistry.set(type, new Set());
+        }
+        return this._eventListenerRegistry.get(type);
+    }
+}