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/2020/ToUint8Clamp.js

@@ -0,0 +1,19 @@
+'use strict';
+
+var ToNumber = require('./ToNumber');
+var floor = require('./floor');
+
+var $isNaN = require('math-intrinsics/isNaN');
+
+// https://262.ecma-international.org/6.0/#sec-touint8clamp
+
+module.exports = function ToUint8Clamp(argument) {
+	var number = ToNumber(argument);
+	if ($isNaN(number) || number <= 0) { return 0; }
+	if (number >= 0xFF) { return 0xFF; }
+	var f = floor(number);
+	if (f + 0.5 < number) { return f + 1; }
+	if (number < f + 0.5) { return f; }
+	if (f % 2 !== 0) { return f + 1; }
+	return f;
+};