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/static/common.js

@@ -62,7 +62,7 @@
 // UI (Layout canvas, Add-a-widget buttons, button-assignment dropdowns).
 const WIDGET_LABELS = {
   photos: 'Photos', calendar: 'Calendar', whiteboard: 'Whiteboard (alpha)', tasks: 'Tasks',
-  static: 'Static image', text: 'Text', weather: 'Weather',
+  static: 'Static image', text: 'Text', weather: 'Weather', battery: 'Battery',
 };
 
 function showStatus(ok, message) {