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>

power/van-battery

@@ -150,13 +150,15 @@ def poweroff(cfg):
         log(f"poweroff failed: {e}", "crit")
 
 
-def write_state(on_batt, cap, armed):
+def write_state(on_batt, cap, armed, warn_levels, shutdown_level):
     STATE_DIR.mkdir(parents=True, exist_ok=True)
     payload = {
         "updated": datetime.now(timezone.utc).isoformat(timespec="seconds"),
         "on_battery": on_batt,
         "capacity": cap,
         "alerted_below": armed,   # lowest level alerted this discharge episode, or null
+        "warn_levels": warn_levels,
+        "shutdown_level": shutdown_level,
     }
     tmp = STATE_PATH.with_suffix(".tmp")
     tmp.write_text(json.dumps(payload))
@@ -216,7 +218,7 @@ def main():
 
         prev_on_batt = on_batt
         try:
-            write_state(on_batt, cap, last_alerted)
+            write_state(on_batt, cap, last_alerted, cfg["warn_levels"], shutdown_level)
         except OSError as e:
             log(f"state write failed: {e}", "warn")
         time.sleep(cfg["poll_interval"])