New PWA icons: slate/blue weather icon, distinct dashboard icon, safe-zone maskable

- Redrawn sun-and-cloud icon in the current palette; artwork sits inside the maskable safe zone
  (the old one was clipped by round/squircle masks and used the old warm colours)
- Separate 'any' and 'maskable' manifest entries; opaque iOS touch icons
- Station Dashboard gets its own chart icon so both installed apps are distinguishable
- Vector favicon + PNG fallback; icons verified served by both manifests

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

app/public/sw.js

@@ -4,7 +4,7 @@ self.addEventListener('message', event => {
   if (event.data === 'skipWaiting') self.skipWaiting();
 });
 
-const CACHE = 'weather-v12';
+const CACHE = 'weather-v13';
 
 // App shell cached on install for offline resilience.
 const PRECACHE = [