fix: meshcore serial connection and battery calculation issues

- Fix _pollUntilConnected overwriting real error with generic timeout
  message — stop polling once backend reaches error state
- Fix battery % calculation: was battery_mv/42 (71% at empty); now
  (battery_mv-3000)/12 for a proper 0-100% LiPo curve
- Improve auto-detect: scan available ports rather than blindly using
  /dev/ttyUSB0; update label to show fallback so users know what to expect
- Add port refresh button (↻) — previously required switching modes
  and back if device was plugged in after panel opened
- Add baud rate selector to serial config strip (default 115200,
  options up to 921600) and wire it through to the connect request

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

utils/meshcore_client.py

@@ -100,7 +100,15 @@ class AsyncWorker:
         cfg = self._config
 
         if isinstance(cfg, SerialConfig):
-            port = cfg.port or "/dev/ttyUSB0"
+            from utils.meshcore import list_serial_ports
+
+            if cfg.port:
+                port = cfg.port
+            else:
+                # Auto-detect: prefer ttyUSB0, then first available port
+                candidates = list_serial_ports()
+                port = next((p for p in candidates if "ttyUSB0" in p), None) or \
+                       (candidates[0] if candidates else "/dev/ttyUSB0")
             self._mc = await MeshCore.create_serial(port=port, baudrate=cfg.baud, debug=False)
             transport, device = "serial", port
         elif isinstance(cfg, TCPConfig):
@@ -205,7 +213,8 @@ class AsyncWorker:
         p = event.payload
         node_id = "self"  # stats_core is always for the local node
         battery_mv = p.get("battery_mv")
-        battery_pct = min(int(battery_mv / 42), 100) if battery_mv else None  # rough: 4200mv = 100%
+        # LiPo linear approximation: 3000mV = 0%, 4200mV = 100%
+        battery_pct = max(0, min(int((battery_mv - 3000) / 12), 100)) if battery_mv else None
         t = MeshcoreTelemetry(
             node_id=node_id,
             timestamp=datetime.now(timezone.utc),