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/picture_target.py

@@ -31,7 +31,8 @@ class PictureTarget:
     def from_dict(cls, data: dict) -> "PictureTarget":
         """Create from dictionary, dispatching to the correct subclass."""
         target_type = data.get("target_type", "wled")
-        if target_type == "wled":
+        # "wled" and "led" both map to WledPictureTarget (backward compat)
+        if target_type in ("wled", "led"):
             from wled_controller.storage.wled_picture_target import WledPictureTarget
             return WledPictureTarget.from_dict(data)
         if target_type == "key_colors":