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

- Add lat/lon/zoom params to geocode() and _retrieve_photon_freetext()
- Update nav_tools.py wrapper to pass through viewport params
- Add /api/geocode handler support for lat/lon/zoom query params
- Add _safe_float() helper for param validation
- Cast zoom to int for Photon compatibility

Allows the frontend to pass current map center/zoom to bias
search results toward the visible area.

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

lib/nav_tools.py

@@ -50,10 +50,10 @@ def _haversine_m(lat1, lon1, lat2, lon2):
     return R * 2 * math.atan2(math.sqrt(a), math.sqrt(1 - a))
 
 
-def geocode(query: str, limit: int = 10):
+def geocode(query: str, limit: int = 10, lat=None, lon=None, zoom=None):
     """Delegate to the structured geocode module. See lib/geocode.py."""
     from . import geocode as geocode_mod
-    return geocode_mod.geocode(query, limit=limit)
+    return geocode_mod.geocode(query, limit=limit, lat=lat, lon=lon, zoom=zoom)
 
 
 def _geocode(query: str):