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/widgets/__init__.py

@@ -36,7 +36,7 @@ Each module in this package exposes:
 
 from __future__ import annotations
 
-from . import calendar, photos, static_image, tasks, text, weather, whiteboard
+from . import battery, calendar, photos, static_image, tasks, text, weather, whiteboard
 
 WIDGET_TYPES = {
     "photos": photos,
@@ -46,4 +46,5 @@ WIDGET_TYPES = {
     "static": static_image,
     "text": text,
     "weather": weather,
+    "battery": battery,
 }