Redesign main app: slim top bar, dynamic sky hero, nowcast, scrub chart, unified palette

- One slate/blue palette across the app, dashboard, account/login pages (data colours
  stay orange = warm, blue = cool on the dashboard)
- Slim sticky top bar with location chip, gear (settings) and ? (learn); 4-tab nav
  (Now, Station, Map, Alerts) with a thumb-reach bottom bar on phones
- Hero sky gradient by condition/time with rain, storm, snow and star effects
  (respects reduced motion); keeps the searched city and badges when temps come from your station
- One-line summary, 'Next 2 hours' bars and rain countdown from 15-minute precipitation,
  rain/cold-aware 'Bring' chips; yesterday + 'vs usual' context from the station
- Scrubbable hourly chart (temp, rain chance, wind), 7-day range bars, AQI gauge,
  details tiles instead of drawers, taller radar with location marker + alert polygons
- Skeleton loading instead of a full-screen spinner, sticky section headers, safe areas
- Server: minutely_15 precipitation, public /api/station/summary

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-v8';
+const CACHE = 'weather-v9';
 
 // App shell cached on install for offline resilience.
 const PRECACHE = [
@@ -13,6 +13,8 @@ const PRECACHE = [
   '/style.css',
   '/station-live.css',
   '/station-live.js',
+  '/app-ui.css',
+  '/app-ui.js',
   '/manifest.json',
   '/icons/icon.svg'
 ];