Add Pattern Templates for Key Colors targets with visual canvas editor

Introduce Pattern Template entity as a reusable rectangle layout that
Key Colors targets reference via pattern_template_id. This replaces
inline rectangle storage with a shared template system.

Backend:
- New PatternTemplate data model, store (JSON persistence), CRUD API
- KC targets now reference pattern_template_id instead of inline rectangles
- ProcessorManager resolves pattern template at KC processing start
- Picture source test endpoint supports capture_duration=0 for single frame
- Delete protection: 409 when template is referenced by a KC target

Frontend:
- Pattern Templates section in Key Colors sub-tab with card UI
- Visual canvas editor with drag-to-move, 8-point resize handles
- Background capture from any picture source for visual alignment
- Precise coordinate list synced bidirectionally with canvas
- Resizable editor container, viewport-constrained modal
- KC target editor uses pattern template dropdown instead of inline rects
- Localization (en/ru) for all new UI elements

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

server/src/wled_controller/api/schemas/picture_sources.py

@@ -60,7 +60,7 @@ class PictureSourceListResponse(BaseModel):
 class PictureSourceTestRequest(BaseModel):
     """Request to test a picture source."""
 
-    capture_duration: float = Field(default=5.0, ge=1.0, le=30.0, description="Duration to capture in seconds")
+    capture_duration: float = Field(default=5.0, ge=0.0, le=30.0, description="Duration to capture in seconds (0 = single frame)")
     border_width: int = Field(default=10, ge=1, le=100, description="Border width in pixels for preview")