fix: BUG-13 — server geocode returns raw coords instead of location names

Root cause: server _geocode_cached() returned None → /api/geocode
returned HTTP 204 with no body. JS frontend treated 204 as success,
r.json() on empty body threw → showed raw-coordinate fallback.

Server fixes:
- _geocode_cached() now always returns a non-empty string (coordinate
  fallback when both Nominatim and reverse_geocoder fail)
- /api/geocode always returns HTTP 200 with {label: '...'} (never 204)
- /api/geocode/batch always includes every coord key in result dict
- Nominatim call: jsonv2 + accept-language=en (aligned with Pi)
- reverse_geocoder: mode=1 (aligned with Pi)

Pi alignment:
- _geocode_cached() now checks for empty reverse_geocoder label before
  returning, falling through to coordinate string (same guard as server)

app.py

@@ -167,7 +167,8 @@ import urllib.request
 
 @functools.lru_cache(maxsize=512)
 def _geocode_cached(lat4, lon4):
-    """Try Nominatim first (street-level), fall back to reverse_geocoder (city)."""
+    """Try Nominatim first (street-level), fall back to reverse_geocoder (city),
+    then raw coordinates. Always returns a non-empty label string."""
     lat = lat4 / 10000.0
     lon = lon4 / 10000.0
     try:
@@ -188,7 +189,10 @@ def _geocode_cached(lat4, lon4):
     results = rg.search([(lat, lon)], mode=1, verbose=False)
     if results:
         r = results[0]
-        return ", ".join(p for p in [r.get("name", ""), r.get("admin1", "")] if p)
+        label = ", ".join(p for p in [r.get("name", ""), r.get("admin1", "")] if p)
+        if label:
+            return label
+    # Last resort — raw coordinates so the frontend never gets an empty label
     return f"{lat:.5f}, {lon:.5f}"
 
 @app.route('/api/geocode')