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/api_widgets.py

@@ -54,6 +54,7 @@ from ..models import (
 )
 from ..text_content import has_text, parse_rich_text
 from ..widgets import WIDGET_TYPES
+from ..widgets import battery as battery_widget
 from ..widgets import text as text_widget
 from .common import (
     calendar_sources_for_widget,
@@ -291,6 +292,8 @@ def api_widget_config_save(
     weather_units: str | None = Form(None),
     weather_hourly_interval_hours: int | None = Form(None),
     weather_daily_days: int | None = Form(None),
+    # battery
+    battery_mode: str | None = Form(None),
 ):
     """Every field optional -- same partial-update, form-urlencoded
     convention as the old frame-level api_config_save, now scoped to one
@@ -417,6 +420,10 @@ def api_widget_config_save(
                 wcfg.hourly_interval_hours = max(1, min(24, weather_hourly_interval_hours))
             if weather_daily_days is not None:
                 wcfg.daily_days = max(1, min(14, weather_daily_days))
+    elif widget.widget_type == "battery":
+        with widget_locked(db, frame.id, widget.id) as (_, _, bcfg):
+            if battery_mode is not None:
+                bcfg.mode = battery_mode if battery_mode in ("compact", "detailed") else "detailed"
     with frame_locked(db, frame.id) as cfg:
         cfg.stats_config_saves += 1
     return {"status": "saved"}
@@ -1059,6 +1066,25 @@ def api_widget_preview_text(
     return Response(content=png, media_type="image/png")
 
 
+# --- Battery: preview --------------------------------------------------------
+
+@router.get("/api/frames/{frame_id}/widgets/{widget_id}/preview/battery")
+def api_widget_preview_battery(
+    frame_widget: tuple[Frame, Widget] = Depends(require_widget_view), db: Session = Depends(get_db)
+):
+    """Unlike every other preview endpoint, there's no "not configured
+    yet" 400 case -- the content is frame-level state (battery_percent)
+    that either exists or doesn't, and render() already degrades to a
+    "No reports yet" placeholder either way, same as a live device
+    render would."""
+    frame, widget = frame_widget
+    _require_widget_type(widget, "battery")
+    png = battery_widget.render_preview_png(
+        db, frame, widget, orientation=frame.orientation, palette_rgb=frame.palette_rgb
+    )
+    return Response(content=png, media_type="image/png")
+
+
 # --- Whiteboard: source/preview ------------------------------------------
 
 class WhiteboardSourceRequest(BaseModel):