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/vocabularies/jtd/nullable.ts

@@ -0,0 +1,21 @@
+import type {KeywordCxt} from "../../compile/validate"
+import {_, not, nil, Code, Name} from "../../compile/codegen"
+
+export function checkNullable(
+  {gen, data, parentSchema}: KeywordCxt,
+  cond: Code = nil
+): [Name, Code] {
+  const valid = gen.name("valid")
+  if (parentSchema.nullable) {
+    gen.let(valid, _`${data} === null`)
+    cond = not(valid)
+  } else {
+    gen.let(valid, false)
+  }
+  return [valid, cond]
+}
+
+export function checkNullableObject(cxt: KeywordCxt, cond: Code): [Name, Code] {
+  const [valid, cond_] = checkNullable(cxt, cond)
+  return [valid, _`${cond_} && typeof ${cxt.data} == "object" && !Array.isArray(${cxt.data})`]
+}