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/@surma/rollup-plugin-off-main-thread/Dockerfile

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+FROM selenium/node-chrome:latest
+
+USER root
+
+RUN apt-get update -qqy \
+  && rm -rf /var/lib/apt/lists/* /var/cache/apt/* \
+  && rm /bin/sh && ln -s /bin/bash /bin/sh \
+  && chown seluser /usr/local
+
+ENV NVM_DIR /usr/local/nvm
+RUN mkdir -p $NVM_DIR \
+  && wget -qO- https://raw.githubusercontent.com/creationix/nvm/v0.35.2/install.sh | bash \
+  && source $NVM_DIR/nvm.sh \
+  && nvm install v12
+
+ENV CHROME_BIN /opt/google/chrome/chrome
+ENV INSIDE_DOCKER=1
+
+WORKDIR /usr/src
+ENTRYPOINT source $NVM_DIR/nvm.sh && npm i && npm test