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/en.ts

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+/** English strings — default locale. Ported from res/values/strings.xml. */
+export const en: Record<string, string> = {
+  app_name: 'Tilfluktsrom',
+
+  // Status
+  status_ready: 'Ready',
+  status_loading: 'Loading shelter data\u2026',
+  status_updating: 'Updating\u2026',
+  status_offline: 'Offline mode',
+  status_shelters_loaded: '%d shelters loaded',
+  status_no_location: 'Waiting for GPS\u2026',
+  status_caching_map: 'Caching map for offline use\u2026',
+
+  // Loading
+  loading_shelters: 'Downloading shelter data\u2026',
+  loading_map: 'Caching map tiles\u2026',
+  loading_map_explanation:
+    'Preparing offline map.\nThe map will scroll briefly to cache your surroundings.',
+  loading_first_time: 'Setting up for first use\u2026',
+
+  // Shelter info
+  shelter_capacity: '%d places',
+  shelter_room_nr: 'Room %d',
+  nearest_shelter: 'Nearest shelter',
+  no_shelters: 'No shelter data available',
+
+  // Actions
+  action_refresh: 'Refresh data',
+  action_toggle_view: 'Toggle map/compass view',
+  action_skip: 'Skip',
+  action_cache_ok: 'Cache map',
+  action_cache_now: 'Cache now',
+  warning_no_map_cache: 'No offline map cached. Map requires internet.',
+
+  // Permissions
+  permission_location_title: 'Location permission required',
+  permission_location_message:
+    'This app needs your location to find the nearest shelter. Please grant location access.',
+  permission_denied:
+    'Location permission denied. The app cannot find nearby shelters without it.',
+
+  // Errors
+  error_download_failed:
+    'Could not download shelter data. Check your internet connection.',
+  error_no_data_offline:
+    'No cached data available. Connect to the internet to download shelter data.',
+  update_success: 'Shelter data updated',
+  update_failed: 'Update failed \u2014 using cached data',
+};

pwa/src/i18n/i18n.ts

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+/**
+ * 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;
+}

pwa/src/i18n/nb.ts

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+/** Norwegian Bokm\u00e5l strings. Ported from res/values-nb/strings.xml. */
+export const nb: Record<string, string> = {
+  app_name: 'Tilfluktsrom',
+
+  status_ready: 'Klar',
+  status_loading: 'Laster tilfluktsromdata\u2026',
+  status_updating: 'Oppdaterer\u2026',
+  status_offline: 'Frakoblet modus',
+  status_shelters_loaded: '%d tilfluktsrom lastet',
+  status_no_location: 'Venter p\u00e5 GPS\u2026',
+  status_caching_map: 'Lagrer kart for frakoblet bruk\u2026',
+
+  loading_shelters: 'Laster ned tilfluktsromdata\u2026',
+  loading_map: 'Lagrer kartfliser\u2026',
+  loading_map_explanation:
+    'Forbereder frakoblet kart.\nKartet vil rulle kort for \u00e5 lagre omgivelsene dine.',
+  loading_first_time: 'Gj\u00f8r klar for f\u00f8rste gangs bruk\u2026',
+
+  shelter_capacity: '%d plasser',
+  shelter_room_nr: 'Rom %d',
+  nearest_shelter: 'N\u00e6rmeste tilfluktsrom',
+  no_shelters: 'Ingen tilfluktsromdata tilgjengelig',
+
+  action_refresh: 'Oppdater data',
+  action_toggle_view: 'Bytt mellom kart og kompassvisning',
+  action_skip: 'Hopp over',
+  action_cache_ok: 'Lagre kart',
+  action_cache_now: 'Lagre n\u00e5',
+  warning_no_map_cache:
+    'Ingen frakoblet kart lagret. Kartet krever internett.',
+
+  permission_location_title: 'Posisjonstillatelse kreves',
+  permission_location_message:
+    'Denne appen trenger din posisjon for \u00e5 finne n\u00e6rmeste tilfluktsrom. Vennligst gi tilgang til posisjon.',
+  permission_denied:
+    'Posisjonstillatelse avsl\u00e5tt. Appen kan ikke finne tilfluktsrom i n\u00e6rheten uten den.',
+
+  error_download_failed:
+    'Kunne ikke laste ned tilfluktsromdata. Sjekk internettforbindelsen.',
+  error_no_data_offline:
+    'Ingen lagrede data tilgjengelig. Koble til internett for \u00e5 laste ned tilfluktsromdata.',
+  update_success: 'Tilfluktsromdata oppdatert',
+  update_failed: 'Oppdatering mislyktes \u2014 bruker lagrede data',
+};

pwa/src/i18n/nn.ts

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+/** Norwegian Nynorsk strings. Ported from res/values-nn/strings.xml. */
+export const nn: Record<string, string> = {
+  app_name: 'Tilfluktsrom',
+
+  status_ready: 'Klar',
+  status_loading: 'Lastar tilfluktsromdata\u2026',
+  status_updating: 'Oppdaterer\u2026',
+  status_offline: 'Fr\u00e5kopla modus',
+  status_shelters_loaded: '%d tilfluktsrom lasta',
+  status_no_location: 'Ventar p\u00e5 GPS\u2026',
+  status_caching_map: 'Lagrar kart for fr\u00e5kopla bruk\u2026',
+
+  loading_shelters: 'Lastar ned tilfluktsromdata\u2026',
+  loading_map: 'Lagrar kartfliser\u2026',
+  loading_map_explanation:
+    'F\u00f8rebur fr\u00e5kopla kart.\nKartet vil rulle kort for \u00e5 lagre omgjevnadene dine.',
+  loading_first_time: 'Gjer klar for fyrste gongs bruk\u2026',
+
+  shelter_capacity: '%d plassar',
+  shelter_room_nr: 'Rom %d',
+  nearest_shelter: 'N\u00e6raste tilfluktsrom',
+  no_shelters: 'Ingen tilfluktsromdata tilgjengeleg',
+
+  action_refresh: 'Oppdater data',
+  action_toggle_view: 'Byt mellom kart og kompassvising',
+  action_skip: 'Hopp over',
+  action_cache_ok: 'Lagre kart',
+  action_cache_now: 'Lagre no',
+  warning_no_map_cache:
+    'Ingen fr\u00e5kopla kart lagra. Kartet krev internett.',
+
+  permission_location_title: 'Posisjonsløyve krevst',
+  permission_location_message:
+    'Denne appen treng posisjonen din for \u00e5 finne n\u00e6raste tilfluktsrom. Ver venleg og gje tilgang til posisjon.',
+  permission_denied:
+    'Posisjonsløyve avsl\u00e5tt. Appen kan ikkje finne tilfluktsrom i n\u00e6rleiken utan det.',
+
+  error_download_failed:
+    'Kunne ikkje laste ned tilfluktsromdata. Sjekk internettilkoplinga.',
+  error_no_data_offline:
+    'Ingen lagra data tilgjengeleg. Kopla til internett for \u00e5 laste ned tilfluktsromdata.',
+  update_success: 'Tilfluktsromdata oppdatert',
+  update_failed: 'Oppdatering mislukkast \u2014 brukar lagra data',
+};