Add battery widget (device's own last-reported level, no live upstream)

Shows Frame.battery_percent/battery_as_of, already set by every device
wake-on-battery report, plus routers/common.py's existing
battery_estimate_s time-remaining estimate -- nothing new to fetch or
cache. Compact (icon + percent) or detailed (+ estimate, last report
age) display mode. No button actions.

server/app/models.py

@@ -433,7 +433,7 @@ class Widget(Base):
 
     id: Mapped[int] = mapped_column(primary_key=True)
     frame_id: Mapped[int] = mapped_column(ForeignKey("frames.id", ondelete="CASCADE"))
-    widget_type: Mapped[str] = mapped_column(String)  # "photos" | "calendar" | "whiteboard" | "tasks" | "static" | "text" | "weather"
+    widget_type: Mapped[str] = mapped_column(String)  # "photos" | "calendar" | "whiteboard" | "tasks" | "static" | "text" | "weather" | "battery"
     x: Mapped[int] = mapped_column(Integer)
     y: Mapped[int] = mapped_column(Integer)
     w: Mapped[int] = mapped_column(Integer)
@@ -643,6 +643,22 @@ class StaticWidgetConfig(Base):
     display_mode: Mapped[str] = mapped_column(String, default="crop_fill")
 
 
+class BatteryWidgetConfig(Base):
+    """One battery widget's display settings -- another no-live-upstream
+    type like StaticWidgetConfig/TextWidgetConfig, just showing existing
+    frame-level state (Frame.battery_percent/battery_as_of, already set
+    by routers/device.py's frame_battery on every device report) instead
+    of anything the widget itself fetches or the user authors. `mode`
+    "compact" is icon + percent only; "detailed" (default) adds the
+    routers.common.battery_estimate_s time-remaining estimate and the
+    last report's age."""
+
+    __tablename__ = "battery_widget_configs"
+
+    widget_id: Mapped[int] = mapped_column(ForeignKey("widgets.id", ondelete="CASCADE"), primary_key=True)
+    mode: Mapped[str] = mapped_column(String, default="detailed")  # compact | detailed
+
+
 # widget_type -> its per-type extension table, keyed by widget_id. Used
 # by db.widget_locked() to resolve the right config row without importing
 # app/widgets/'s heavier render/action registry just for this lookup.
@@ -652,6 +668,7 @@ WIDGET_CONFIG_MODELS: dict[str, type] = {
     "whiteboard": WhiteboardWidgetConfig,
     "tasks": TaskWidgetConfig,
     "static": StaticWidgetConfig,
+    "battery": BatteryWidgetConfig,
     "text": TextWidgetConfig,
     "weather": WeatherWidgetConfig,
 }