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/widget_dialog_battery.js

@@ -0,0 +1,37 @@
+// Battery widget dialog: display-mode setting and the rendered preview.
+// Not a page-load script -- frame_layout.js fetches this widget's dialog
+// HTML fragment, injects it into the shared <dialog>, points
+// window.FRAME_API at this specific widget
+// (/api/frames/{id}/widgets/{widget_id}), then calls initBatteryDialog().
+
+function loadBatteryPreview() {
+  document.getElementById('battery-preview').src = `${window.FRAME_API}/preview/battery?_=${Date.now()}`;
+}
+
+function initBatteryDialog() {
+  document.getElementById('battery-config-form').addEventListener('submit', async (e) => {
+    e.preventDefault();
+    const body = new URLSearchParams({
+      battery_mode: document.getElementById('battery_mode').value,
+    });
+    try {
+      const resp = await fetch(`${window.FRAME_API}/config`, {
+        method: 'POST',
+        headers: { 'Content-Type': 'application/x-www-form-urlencoded' },
+        body,
+      });
+      if (!resp.ok) throw new Error(await apiError(resp));
+      showStatus(true, 'Saved.');
+      loadBatteryPreview();
+    } catch (e) {
+      showStatus(false, e.message);
+    }
+  });
+
+  document.getElementById('battery-preview-refresh').addEventListener('click', loadBatteryPreview);
+  loadBatteryPreview();
+}
+
+function closeBatteryDialog() {
+  // Nothing to tear down -- no poll interval, no upload state.
+}