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/main.py

@@ -154,6 +154,7 @@ async def lifespan(app: FastAPI):
                 device_url=device.url,
                 led_count=device.led_count,
                 calibration=device.calibration,
+                device_type=device.device_type,
             )
             logger.info(f"Registered device: {device.name} ({device.id})")
         except Exception as e: