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/object.assign/implementation.js

@@ -0,0 +1,46 @@
+'use strict';
+
+// modified from https://github.com/es-shims/es6-shim
+var objectKeys = require('object-keys');
+var hasSymbols = require('has-symbols/shams')();
+var callBound = require('call-bound');
+var $Object = require('es-object-atoms');
+var $push = callBound('Array.prototype.push');
+var $propIsEnumerable = callBound('Object.prototype.propertyIsEnumerable');
+var originalGetSymbols = hasSymbols ? $Object.getOwnPropertySymbols : null;
+
+// eslint-disable-next-line no-unused-vars
+module.exports = function assign(target, source1) {
+	if (target == null) { throw new TypeError('target must be an object'); }
+	var to = $Object(target); // step 1
+	if (arguments.length === 1) {
+		return to; // step 2
+	}
+	for (var s = 1; s < arguments.length; ++s) {
+		var from = $Object(arguments[s]); // step 3.a.i
+
+		// step 3.a.ii:
+		var keys = objectKeys(from);
+		var getSymbols = hasSymbols && ($Object.getOwnPropertySymbols || originalGetSymbols);
+		if (getSymbols) {
+			var syms = getSymbols(from);
+			for (var j = 0; j < syms.length; ++j) {
+				var key = syms[j];
+				if ($propIsEnumerable(from, key)) {
+					$push(keys, key);
+				}
+			}
+		}
+
+		// step 3.a.iii:
+		for (var i = 0; i < keys.length; ++i) {
+			var nextKey = keys[i];
+			if ($propIsEnumerable(from, nextKey)) { // step 3.a.iii.2
+				var propValue = from[nextKey]; // step 3.a.iii.2.a
+				to[nextKey] = propValue; // step 3.a.iii.2.b
+			}
+		}
+	}
+
+	return to; // step 4
+};