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/routers/frame_pages.py

@@ -29,6 +29,7 @@ from ..image_pipeline import (
     palette_to_hex,
 )
 from ..models import (
+    BatteryWidgetConfig,
     CalendarWidgetConfig,
     Frame,
     FrameCalendar,
@@ -274,4 +275,10 @@ def widget_dialog(frame_id: int, widget_id: int, request: Request, db: Session =
             "weather_provider_labels": weather.PROVIDER_LABELS,
         })
 
+    if widget.widget_type == "battery":
+        battery_cfg = db.get(BatteryWidgetConfig, widget.id)
+        return templates.TemplateResponse("_widget_dialog_battery.html", {
+            "request": request, "frame": frame, "widget": widget, "battery_cfg": battery_cfg,
+        })
+
     raise HTTPException(400, f"Unknown widget type: {widget.widget_type}")