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/src/i18n/i18n.ts

@@ -0,0 +1,49 @@
+/**
+ * Minimal i18n system: detects locale from navigator.languages,
+ * provides t(key, ...args) for string lookup with %d/%s substitution.
+ */
+
+import { en } from './en';
+import { nb } from './nb';
+import { nn } from './nn';
+
+const locales: Record<string, Record<string, string>> = { en, nb, nn };
+
+let currentLocale = 'en';
+
+/** Detect and set locale from browser preferences. */
+export function initLocale(): void {
+  const langs = navigator.languages ?? [navigator.language];
+  for (const lang of langs) {
+    const code = lang.toLowerCase().split('-')[0];
+    if (code in locales) {
+      currentLocale = code;
+      return;
+    }
+    // nb and nn both start with "n" — also match "no" as Bokmål
+    if (code === 'no') {
+      currentLocale = 'nb';
+      return;
+    }
+  }
+}
+
+/** Get current locale code. */
+export function getLocale(): string {
+  return currentLocale;
+}
+
+/**
+ * Translate a string key, substituting %d and %s with provided arguments.
+ * Falls back to English, then to the raw key.
+ */
+export function t(key: string, ...args: (string | number)[]): string {
+  let str = locales[currentLocale]?.[key] ?? locales.en[key] ?? key;
+
+  // Replace %d and %s placeholders sequentially
+  for (const arg of args) {
+    str = str.replace(/%[ds]/, String(arg));
+  }
+
+  return str;
+}