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/es-abstract/2021/IsValidIntegerIndex.js

@@ -0,0 +1,30 @@
+'use strict';
+
+var $TypeError = require('es-errors/type');
+var isNegativeZero = require('math-intrinsics/isNegativeZero');
+
+var IsDetachedBuffer = require('./IsDetachedBuffer');
+
+var isInteger = require('math-intrinsics/isInteger');
+var typedArrayBuffer = require('typed-array-buffer');
+
+// https://262.ecma-international.org/12.0/#sec-isvalidintegerindex
+
+module.exports = function IsValidIntegerIndex(O, index) {
+	// Assert: O is an Integer-Indexed exotic object.
+	var buffer = typedArrayBuffer(O); // step 1
+
+	if (typeof index !== 'number') {
+		throw new $TypeError('Assertion failed: Type(index) is not Number');
+	}
+
+	if (IsDetachedBuffer(buffer)) { return false; } // step 2
+
+	if (!isInteger(index)) { return false; } // step 3
+
+	if (isNegativeZero(index)) { return false; } // step 4
+
+	if (index < 0 || index >= O.length) { return false; } // step 5
+
+	return true; // step 6
+};