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/ui/loading-overlay.ts

@@ -0,0 +1,57 @@
+/**
+ * Loading overlay: spinner + message + OK/Skip buttons.
+ * Same flow as Android: prompt before map caching, user can skip.
+ */
+
+/** Show the loading overlay with a message and optional spinner. */
+export function showLoading(message: string, showSpinner = true): void {
+  const overlay = document.getElementById('loading-overlay')!;
+  const text = document.getElementById('loading-text')!;
+  const spinner = document.getElementById('loading-spinner')!;
+  const buttonRow = document.getElementById('loading-button-row')!;
+
+  text.textContent = message;
+  spinner.style.display = showSpinner ? 'block' : 'none';
+  buttonRow.style.display = 'none';
+  overlay.style.display = 'flex';
+}
+
+/** Show the cache prompt (OK / Skip buttons, no spinner). */
+export function showCachePrompt(
+  message: string,
+  onOk: () => void,
+  onSkip: () => void,
+): void {
+  const overlay = document.getElementById('loading-overlay')!;
+  const text = document.getElementById('loading-text')!;
+  const spinner = document.getElementById('loading-spinner')!;
+  const buttonRow = document.getElementById('loading-button-row')!;
+  const okBtn = document.getElementById('loading-ok-btn')!;
+  const skipBtn = document.getElementById('loading-skip-btn')!;
+
+  text.textContent = message;
+  spinner.style.display = 'none';
+  buttonRow.style.display = 'flex';
+  overlay.style.display = 'flex';
+
+  okBtn.onclick = () => {
+    hideLoading();
+    onOk();
+  };
+  skipBtn.onclick = () => {
+    hideLoading();
+    onSkip();
+  };
+}
+
+/** Update loading text (e.g. progress). */
+export function updateLoadingText(message: string): void {
+  const text = document.getElementById('loading-text');
+  if (text) text.textContent = message;
+}
+
+/** Hide the loading overlay. */
+export function hideLoading(): void {
+  const overlay = document.getElementById('loading-overlay');
+  if (overlay) overlay.style.display = 'none';
+}