cockpit: battery/power card; van-battery publishes thresholds in state

New "Battery / Power" card shows mains vs battery, charge % with a severity-
colored bar (only bites while on battery), the last alert level, and the
configured warn/shutdown thresholds. van-battery now includes warn_levels +
shutdown_level in /run/van-battery/state.json so the card stays self-describing
instead of hardcoding the numbers.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

cockpit/vanrouter/index.html

@@ -28,6 +28,11 @@
             <div id="thermal"></div>
         </div>
 
+        <div class="card">
+            <h3>Battery / Power</h3>
+            <div id="battery"></div>
+        </div>
+
         <div class="card">
             <h3>WAN Failover</h3>
             <div id="failover"></div>