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/wled_picture_target.py

@@ -1,4 +1,4 @@
-"""WLED picture target — streams a picture source to a WLED device."""
+"""LED picture target — streams a picture source to an LED device."""
 
 from dataclasses import dataclass, field
 from datetime import datetime
@@ -10,7 +10,7 @@ from wled_controller.storage.picture_target import PictureTarget
 
 @dataclass
 class WledPictureTarget(PictureTarget):
-    """WLED picture target — streams a picture source to a WLED device."""
+    """LED picture target — streams a picture source to an LED device."""
 
     device_id: str = ""
     picture_source_id: str = ""
@@ -24,7 +24,6 @@ class WledPictureTarget(PictureTarget):
         d["settings"] = {
             "display_index": self.settings.display_index,
             "fps": self.settings.fps,
-            "border_width": self.settings.border_width,
             "brightness": self.settings.brightness,
             "gamma": self.settings.gamma,
             "saturation": self.settings.saturation,
@@ -44,7 +43,6 @@ class WledPictureTarget(PictureTarget):
         settings = ProcessingSettings(
             display_index=settings_data.get("display_index", 0),
             fps=settings_data.get("fps", 30),
-            border_width=settings_data.get("border_width", 10),
             brightness=settings_data.get("brightness", 1.0),
             gamma=settings_data.get("gamma", 2.2),
             saturation=settings_data.get("saturation", 1.0),
@@ -57,7 +55,7 @@ class WledPictureTarget(PictureTarget):
         return cls(
             id=data["id"],
             name=data["name"],
-            target_type=data.get("target_type", "wled"),
+            target_type="led",
             device_id=data.get("device_id", ""),
             picture_source_id=data.get("picture_source_id", ""),
             settings=settings,