Add LED device abstraction layer for multi-controller support

Introduce abstract LEDClient base class with factory pattern so new
LED controller types can plug in alongside WLED. ProcessorManager is
now fully type-agnostic — all device-specific logic (health checks,
state snapshot/restore, fast send) lives behind the LEDClient interface.

- New led_client.py: LEDClient ABC, DeviceHealth, factory functions
- WLEDClient inherits LEDClient, encapsulates WLED health checks and state management
- device_type field on Device storage model (defaults to "wled")
- Rename target_type "wled" → "led" with backward-compat migration
- Frontend: "WLED" tab → "LED", device type badge, type selector in
  add-device modal, device type shown in target device dropdown
- All wled_* API fields renamed to device_* for generic naming

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

server/src/wled_controller/storage/device_store.py

@@ -31,6 +31,7 @@ class Device:
         url: str,
         led_count: int,
         enabled: bool = True,
+        device_type: str = "wled",
         calibration: Optional[CalibrationConfig] = None,
         created_at: Optional[datetime] = None,
         updated_at: Optional[datetime] = None,
@@ -40,6 +41,7 @@ class Device:
         self.url = url
         self.led_count = led_count
         self.enabled = enabled
+        self.device_type = device_type
         self.calibration = calibration or create_default_calibration(led_count)
         self.created_at = created_at or datetime.utcnow()
         self.updated_at = updated_at or datetime.utcnow()
@@ -52,6 +54,7 @@ class Device:
             "url": self.url,
             "led_count": self.led_count,
             "enabled": self.enabled,
+            "device_type": self.device_type,
             "calibration": calibration_to_dict(self.calibration),
             "created_at": self.created_at.isoformat(),
             "updated_at": self.updated_at.isoformat(),
@@ -77,6 +80,7 @@ class Device:
             url=data["url"],
             led_count=data["led_count"],
             enabled=data.get("enabled", True),
+            device_type=data.get("device_type", "wled"),
             calibration=calibration,
             created_at=datetime.fromisoformat(data.get("created_at", datetime.utcnow().isoformat())),
             updated_at=datetime.fromisoformat(data.get("updated_at", datetime.utcnow().isoformat())),
@@ -160,6 +164,7 @@ class DeviceStore:
         name: str,
         url: str,
         led_count: int,
+        device_type: str = "wled",
         calibration: Optional[CalibrationConfig] = None,
     ) -> Device:
         """Create a new device."""
@@ -170,6 +175,7 @@ class DeviceStore:
             name=name,
             url=url,
             led_count=led_count,
+            device_type=device_type,
             calibration=calibration,
         )