feat(search): add viewport bias for location-aware geocoding

- Add mapCenter state to store (lat/lon/zoom)
- Track map center on moveend in MapView
- Pass mapCenter to searchGeocode from SearchBar
- Update searchGeocode API call to include viewport params

Search results now prioritize locations near the current map view.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/store.js

@@ -40,6 +40,10 @@ export const useStore = create((set, get) => ({
   setUserLocation: (loc) => set({ userLocation: loc }),
   setGeoPermission: (p) => set({ geoPermission: p }),
 
+  // ── Map viewport (for search bias) ──
+  mapCenter: null, // { lat, lon, zoom }
+  setMapCenter: (center) => set({ mapCenter: center }),
+
   // ── Mode ──
   mode: 'auto', // 'auto' | 'pedestrian' | 'bicycle'
   setMode: (mode) => set({ mode }),