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/api/routes/picture_targets.py

@@ -65,7 +65,6 @@ def _settings_to_core(schema: ProcessingSettingsSchema) -> ProcessingSettings:
     settings = ProcessingSettings(
         display_index=schema.display_index,
         fps=schema.fps,
-        border_width=schema.border_width,
         interpolation_mode=schema.interpolation_mode,
         brightness=schema.brightness,
         smoothing=schema.smoothing,
@@ -86,7 +85,6 @@ def _settings_to_schema(settings: ProcessingSettings) -> ProcessingSettingsSchem
     return ProcessingSettingsSchema(
         display_index=settings.display_index,
         fps=settings.fps,
-        border_width=settings.border_width,
         interpolation_mode=settings.interpolation_mode,
         brightness=settings.brightness,
         smoothing=settings.smoothing,
@@ -468,7 +466,6 @@ async def update_target_settings(
         new_settings = ProcessingSettings(
             display_index=settings.display_index if 'display_index' in sent else existing.display_index,
             fps=settings.fps if 'fps' in sent else existing.fps,
-            border_width=settings.border_width if 'border_width' in sent else existing.border_width,
             interpolation_mode=settings.interpolation_mode if 'interpolation_mode' in sent else existing.interpolation_mode,
             brightness=settings.brightness if 'brightness' in sent else existing.brightness,
             gamma=existing.gamma,