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-streams/utils/createHeaders.js

@@ -0,0 +1,31 @@
+/*
+  Copyright 2018 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.
+*/
+import '../_version.js';
+/**
+ * This is a utility method that determines whether the current browser supports
+ * the features required to create streamed responses. Currently, it checks if
+ * [`ReadableStream`](https://developer.mozilla.org/en-US/docs/Web/API/ReadableStream/ReadableStream)
+ * is available.
+ *
+ * @private
+ * @param {HeadersInit} [headersInit] If there's no `Content-Type` specified,
+ * `'text/html'` will be used by default.
+ * @return {boolean} `true`, if the current browser meets the requirements for
+ * streaming responses, and `false` otherwise.
+ *
+ * @memberof workbox-streams
+ */
+function createHeaders(headersInit = {}) {
+    // See https://github.com/GoogleChrome/workbox/issues/1461
+    const headers = new Headers(headersInit);
+    if (!headers.has('content-type')) {
+        headers.set('content-type', 'text/html');
+    }
+    return headers;
+}
+export { createHeaders };