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/stackback/README.md

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+# stackback
+
+Returns an array of CallSite objects for a captured stacktrace. Useful if you want to access the frame for an error object.
+
+## use
+
+```javascript
+var stackback = require('stackback');
+
+// error generated from somewhere
+var err = new Error('some sample error');
+
+// stack is an array of CallSite objects
+var stack = stackback(err);
+```
+
+## CallSite object
+
+From the [V8 StackTrace API](https://code.google.com/p/v8/wiki/JavaScriptStackTraceApi)
+
+The structured stack trace is an Array of CallSite objects, each of which represents a stack frame. A CallSite object defines the following methods
+
+getThis: returns the value of this  
+getTypeName: returns the type of this as a string. This is the name of the function stored in the constructor field of this, if available, otherwise the object's [[Class]] internal property.  
+getFunction: returns the current function  
+getFunctionName: returns the name of the current function, typically its name property. If a name property is not available an attempt will be made to try to infer a name from the function's context.  
+getMethodName: returns the name of the property of this or one of its prototypes that holds the current function  
+getFileName: if this function was defined in a script returns the name of the script  
+getLineNumber: if this function was defined in a script returns the current line number  
+getColumnNumber: if this function was defined in a script returns the current column number  
+getEvalOrigin: if this function was created using a call to eval returns a CallSite object representing the location where eval was called  
+isToplevel: is this a toplevel invocation, that is, is this the global object?  
+isEval: does this call take place in code defined by a call to eval?  
+isNative: is this call in native V8 code?  
+isConstructor: is this a constructor call?  
+
+## install
+
+```shell
+npm install stackback
+```