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-array-buffer/index.js

@@ -0,0 +1,43 @@
+'use strict';
+
+var callBind = require('call-bind');
+var callBound = require('call-bound');
+var GetIntrinsic = require('get-intrinsic');
+
+var $ArrayBuffer = GetIntrinsic('%ArrayBuffer%', true);
+/** @type {undefined | ((receiver: ArrayBuffer) => number) | ((receiver: unknown) => never)} */
+var $byteLength = callBound('ArrayBuffer.prototype.byteLength', true);
+var $toString = callBound('Object.prototype.toString');
+
+// in node 0.10, ArrayBuffers have no prototype methods, but have an own slot-checking `slice` method
+var abSlice = !!$ArrayBuffer && !$byteLength && new $ArrayBuffer(0).slice;
+var $abSlice = !!abSlice && callBind(abSlice);
+
+/** @type {import('.')} */
+module.exports = $byteLength || $abSlice
+	? function isArrayBuffer(obj) {
+		if (!obj || typeof obj !== 'object') {
+			return false;
+		}
+		try {
+			if ($byteLength) {
+				// @ts-expect-error no idea why TS can't handle the overload
+				$byteLength(obj);
+			} else {
+				// @ts-expect-error TS chooses not to type-narrow inside a closure
+				$abSlice(obj, 0);
+			}
+			return true;
+		} catch (e) {
+			return false;
+		}
+	}
+	: $ArrayBuffer
+		// in node 0.8, ArrayBuffers have no prototype or own methods, but also no Symbol.toStringTag
+		? function isArrayBuffer(obj) {
+			return $toString(obj) === '[object ArrayBuffer]';
+		}
+		// @ts-expect-error
+		: function isArrayBuffer(obj) { // eslint-disable-line no-unused-vars
+			return false;
+		};