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/.runkit_example.js

@@ -0,0 +1,23 @@
+const Ajv = require("ajv")
+const ajv = new Ajv({allErrors: true})
+
+const schema = {
+  type: "object",
+  properties: {
+    foo: {type: "string"},
+    bar: {type: "number", maximum: 3},
+  },
+  required: ["foo", "bar"],
+  additionalProperties: false,
+}
+
+const validate = ajv.compile(schema)
+
+test({foo: "abc", bar: 2})
+test({foo: 2, bar: 4})
+
+function test(data) {
+  const valid = validate(data)
+  if (valid) console.log("Valid!")
+  else console.log("Invalid: " + ajv.errorsText(validate.errors))
+}