Fix saved layouts silently dropping weather widget settings

LAYOUT_CONFIG_FIELDS never had a "weather" entry, so saving a layout
captured an empty config for any weather widget -- applying it back
(including via hold-to-cycle) reset mode/provider/city/units/etc to
defaults instead of restoring what was configured.

server/app/routers/api_layouts.py

@@ -63,6 +63,10 @@ LAYOUT_CONFIG_FIELDS: dict[str, tuple[str, ...]] = {
     "text": ("content", "font_size", "font_family", "align", "background_color"),
     "whiteboard": ("user_id", "url"),
     "battery": ("mode",),
+    "weather": (
+        "mode", "provider", "units", "city_label", "city_latitude", "city_longitude",
+        "hourly_interval_hours", "daily_days", "cities",
+    ),
 }
 
 # widget_type -> (FrameCalendar|FrameTaskList model, SavedLayoutSource.kind)