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/data/shelter-db.ts

@@ -0,0 +1,73 @@
+/**
+ * IndexedDB wrapper for shelter data using the idb library.
+ * Ported from Room database in the Android app.
+ */
+
+import { openDB, type IDBPDatabase } from 'idb';
+import type { Shelter } from '../types';
+
+const DB_NAME = 'tilfluktsrom';
+const DB_VERSION = 1;
+const SHELTER_STORE = 'shelters';
+const META_STORE = 'metadata';
+
+const META_KEY_LAST_UPDATE = 'lastUpdate';
+
+type TilfluktsromDB = IDBPDatabase;
+
+let dbPromise: Promise<TilfluktsromDB> | null = null;
+
+function getDb(): Promise<TilfluktsromDB> {
+  if (!dbPromise) {
+    dbPromise = openDB(DB_NAME, DB_VERSION, {
+      upgrade(db) {
+        if (!db.objectStoreNames.contains(SHELTER_STORE)) {
+          db.createObjectStore(SHELTER_STORE, { keyPath: 'lokalId' });
+        }
+        if (!db.objectStoreNames.contains(META_STORE)) {
+          db.createObjectStore(META_STORE);
+        }
+      },
+    });
+  }
+  return dbPromise;
+}
+
+/** Get all cached shelters. */
+export async function getAllShelters(): Promise<Shelter[]> {
+  const db = await getDb();
+  return db.getAll(SHELTER_STORE);
+}
+
+/** Check if any shelter data is cached. */
+export async function hasCachedData(): Promise<boolean> {
+  const db = await getDb();
+  const count = await db.count(SHELTER_STORE);
+  return count > 0;
+}
+
+/** Replace all shelter data (clear + insert). */
+export async function replaceShelters(shelters: Shelter[]): Promise<void> {
+  const db = await getDb();
+  const tx = db.transaction(SHELTER_STORE, 'readwrite');
+  await tx.store.clear();
+  for (const shelter of shelters) {
+    await tx.store.put(shelter);
+  }
+  await tx.done;
+
+  // Update last-update timestamp
+  const metaTx = db.transaction(META_STORE, 'readwrite');
+  await metaTx.store.put(Date.now(), META_KEY_LAST_UPDATE);
+  await metaTx.done;
+}
+
+/** Check if cached data is older than the given max age (default 7 days). */
+export async function isDataStale(
+  maxAgeMs = 7 * 24 * 60 * 60 * 1000,
+): Promise<boolean> {
+  const db = await getDb();
+  const lastUpdate = await db.get(META_STORE, META_KEY_LAST_UPDATE);
+  if (lastUpdate == null) return true;
+  return Date.now() - (lastUpdate as number) > maxAgeMs;
+}

pwa/src/data/shelter-repository.ts

@@ -0,0 +1,33 @@
+/**
+ * Repository that manages shelter data: fetches pre-processed JSON,
+ * caches in IndexedDB, and handles staleness checks.
+ *
+ * Unlike the Android app, no ZIP handling or coordinate conversion
+ * is needed at runtime — the data is pre-processed at build time.
+ */
+
+import type { Shelter } from '../types';
+import {
+  getAllShelters,
+  hasCachedData,
+  replaceShelters,
+  isDataStale,
+} from './shelter-db';
+
+const SHELTERS_JSON_PATH = './data/shelters.json';
+
+/** Fetch shelters.json and cache in IndexedDB. Returns success. */
+export async function refreshData(): Promise<boolean> {
+  try {
+    const response = await fetch(SHELTERS_JSON_PATH);
+    if (!response.ok) return false;
+
+    const shelters: Shelter[] = await response.json();
+    await replaceShelters(shelters);
+    return true;
+  } catch {
+    return false;
+  }
+}
+
+export { getAllShelters, hasCachedData, isDataStale };