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/ajv/lib/compile/ref_error.ts

@@ -0,0 +1,13 @@
+import {resolveUrl, normalizeId, getFullPath} from "./resolve"
+import type {UriResolver} from "../types"
+
+export default class MissingRefError extends Error {
+  readonly missingRef: string
+  readonly missingSchema: string
+
+  constructor(resolver: UriResolver, baseId: string, ref: string, msg?: string) {
+    super(msg || `can't resolve reference ${ref} from id ${baseId}`)
+    this.missingRef = resolveUrl(resolver, baseId, ref)
+    this.missingSchema = normalizeId(getFullPath(resolver, this.missingRef))
+  }
+}