feat: battery-aware itinerary nudge + PWA / offline support

Battery nudge:
- lib/batteryPlan: project SoC through the planned stops using current
  Tesla battery (or 80% default), 85% of rated range, and conservative
  per-stop charge rates (Supercharger 1.5%/min, dest charger 0.4%/min).
- LegRow shows "arrive X%" plus a tone (red/amber/transparent) and a
  TIGHT / WON'T REACH chip if the projected arrival is below 15%/5%.
- Top-of-itinerary banner when any leg is danger or warning, naming the
  current SoC source (live Tesla vs assumed 80%).

PWA / offline:
- vite-plugin-pwa with autoUpdate strategy; ServiceWorker registered in
  main.tsx (prod-only).
- Workbox cache strategies:
    map tiles      CacheFirst, 30 days, 800 entries
    nominatim/OSRM NetworkFirst with cached fallback
    /api/*         NetworkFirst, short TTL, last response stays usable
- App shell precached so the planner renders offline; navigateFallback
  ensures deep links serve index.html without a network round trip.
- manifest.webmanifest + theme-color + favicon.svg (replaces dead
  /tesla-icon.svg reference). Installable to home screen.
- Sonner toasts on offline-ready and update-available.

client/package.json

@@ -11,6 +11,7 @@
   },
   "dependencies": {
     "@tonycodes/auth-react": "^1.4.0",
+    "dompurify": "^3.2.4",
     "framer-motion": "^11.0.0",
     "leaflet": "^1.9.4",
     "lucide-react": "^0.303.0",
@@ -20,10 +21,10 @@
     "react-resizable-panels": "^2.1.7",
     "react-router-dom": "^6.21.1",
     "sonner": "^1.4.0",
-    "tailwind-merge": "^2.2.0",
-    "dompurify": "^3.2.4"
+    "tailwind-merge": "^2.2.0"
   },
   "devDependencies": {
+    "@types/dompurify": "^3.0.5",
     "@types/leaflet": "^1.9.14",
     "@types/node": "^20.10.0",
     "@types/react": "^18.2.47",
@@ -34,6 +35,7 @@
     "tailwindcss": "^3.4.0",
     "typescript": "^5.3.2",
     "vite": "^5.0.8",
-    "@types/dompurify": "^3.0.5"
+    "vite-plugin-pwa": "^1.3.0",
+    "workbox-window": "^7.4.1"
   }
 }