Add Picture Streams architecture with postprocessing templates and stream test UI

Introduce Picture Stream abstraction that separates the capture pipeline into
composable layers: raw streams (display + capture engine + FPS) and processed
streams (source stream + postprocessing template). Devices reference a picture
stream instead of managing individual capture settings.

- Add PictureStream and PostprocessingTemplate data models and stores
- Add CRUD API endpoints for picture streams and postprocessing templates
- Add stream chain resolution in ProcessorManager for start_processing
- Add picture stream test endpoint with postprocessing preview support
- Add Stream Settings modal with border_width and interpolation_mode controls
- Add stream test modal with capture preview and performance metrics
- Add full frontend: Picture Streams tab, Processing Templates tab, stream
  selector on device cards, test buttons on stream cards
- Add localization keys for all new features (en, ru)
- Migrate existing devices to picture streams on startup

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

server/src/wled_controller/storage/template.py

@@ -13,7 +13,6 @@ class CaptureTemplate:
     name: str
     engine_type: str
     engine_config: Dict[str, Any]
-    is_default: bool
     created_at: datetime
     updated_at: datetime
     description: Optional[str] = None
@@ -29,7 +28,6 @@ class CaptureTemplate:
             "name": self.name,
             "engine_type": self.engine_type,
             "engine_config": self.engine_config,
-            "is_default": self.is_default,
             "created_at": self.created_at.isoformat(),
             "updated_at": self.updated_at.isoformat(),
             "description": self.description,
@@ -50,7 +48,6 @@ class CaptureTemplate:
             name=data["name"],
             engine_type=data["engine_type"],
             engine_config=data.get("engine_config", {}),
-            is_default=data.get("is_default", False),
             created_at=datetime.fromisoformat(data["created_at"])
             if isinstance(data.get("created_at"), str)
             else data.get("created_at", datetime.utcnow()),