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/templates/_widget_dialog_battery.html

@@ -0,0 +1,23 @@
+<h2 class="dialog-title">Battery widget</h2>
+
+<section class="card">
+  <h2 class="card-title">Settings</h2>
+  <p class="sub">Shows this frame's own last-reported battery level --
+    nothing to configure beyond how much detail to show.</p>
+  <form id="battery-config-form">
+    <label>Display mode
+      <select id="battery_mode">
+        <option value="compact" {% if battery_cfg and battery_cfg.mode == 'compact' %}selected{% endif %}>Compact (icon + percent only)</option>
+        <option value="detailed" {% if not battery_cfg or battery_cfg.mode == 'detailed' %}selected{% endif %}>Detailed (+ estimated time left, last report)</option>
+      </select>
+    </label>
+    <button type="submit">Save</button>
+  </form>
+</section>
+
+<section class="card" style="margin-top: 20px;">
+  <h2 class="card-title">Preview</h2>
+  <p class="sub">How this widget currently renders.</p>
+  <img class="preview-img" id="battery-preview" alt="Battery widget preview">
+  <button type="button" class="secondary" id="battery-preview-refresh">Refresh now</button>
+</section>