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/leaflet.offline/.eslintrc.js

@@ -0,0 +1,28 @@
+module.exports = {
+  ignorePatterns: ['/dist'],
+  env: {
+    browser: true,
+  },
+  extends: ['airbnb-base', 'prettier'],
+  parserOptions: {
+    project: './tsconfig.json',
+  },
+  overrides: [
+    {
+      files: ['src/**/*.ts', 'test/**/*.ts'],
+      extends: ['airbnb-base', 'airbnb-typescript/base', 'prettier'],
+      rules: {
+        'import/no-extraneous-dependencies': [
+          'error',
+          {
+            devDependencies: true,
+            optionalDependencies: false,
+            peerDependencies: false,
+          },
+        ],
+        'no-underscore-dangle': 0,
+        'no-return-assign': ['error', 'except-parens'],
+      },
+    },
+  ],
+};