Fix weather city name getting wiped on every settings save

The city text field isn't pre-filled with the current value (unlike
the manual lat/lon fields below it), so it's blank on every
settings-only save -- changing forecast_days, a display toggle, etc.
That fell into the branch that unconditionally set "city": "", so the
resolved city name silently disappeared on every save even though
lat/lon round-tripped fine (those fields *are* pre-filled).

Now only clears the city label when the submitted coordinates
actually differ from what's stored. Verified both directions end to
end: city survives a settings-only save, and still clears correctly
when coordinates are genuinely changed.

web/app.py

@@ -98,10 +98,20 @@ def create_app(config, screens: dict, scheduler) -> Flask:
                     scheduler.invalidate_screen("weather")
                     return redirect(url_for("settings_weather", saved=1))
             else:
+                # The city text field isn't pre-filled (unlike the manual
+                # lat/lon fields below it), so it's blank on every
+                # settings-only save -- toggles, forecast_days, etc. Only
+                # clear the resolved city label when the coordinates
+                # actually changed; otherwise this would silently wipe the
+                # city on every save even though lat/lon round-trip fine.
+                current = config.get_screen_settings("weather")
+                new_lat = float(lat_raw) if lat_raw else None
+                new_lon = float(lon_raw) if lon_raw else None
+                coords_unchanged = new_lat == current.get("latitude") and new_lon == current.get("longitude")
                 config.update_screen_settings("weather", {
-                    "city": "",
-                    "latitude": float(lat_raw) if lat_raw else None,
-                    "longitude": float(lon_raw) if lon_raw else None,
+                    "city": current.get("city", "") if coords_unchanged else "",
+                    "latitude": new_lat,
+                    "longitude": new_lon,
                     "forecast_days": forecast_days,
                     "update_minutes": update_minutes,
                     **display_toggles,