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/2019/UTF16Encoding.js

@@ -0,0 +1,25 @@
+'use strict';
+
+var GetIntrinsic = require('get-intrinsic');
+
+var $TypeError = require('es-errors/type');
+var $fromCharCode = GetIntrinsic('%String.fromCharCode%');
+
+var floor = require('./floor');
+var modulo = require('./modulo');
+
+var isCodePoint = require('../helpers/isCodePoint');
+
+// https://262.ecma-international.org/7.0/#sec-utf16encoding
+
+module.exports = function UTF16Encoding(cp) {
+	if (!isCodePoint(cp)) {
+		throw new $TypeError('Assertion failed: `cp` must be >= 0 and <= 0x10FFFF');
+	}
+	if (cp <= 65535) {
+		return $fromCharCode(cp);
+	}
+	var cu1 = $fromCharCode(floor((cp - 65536) / 1024) + 0xD800);
+	var cu2 = $fromCharCode(modulo(cp - 65536, 1024) + 0xDC00);
+	return cu1 + cu2;
+};