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/storage/__init__.py

@@ -1,7 +1,8 @@
 """Storage layer for device and configuration persistence."""
 
 from .device_store import DeviceStore
+from .pattern_template_store import PatternTemplateStore
 from .picture_source_store import PictureSourceStore
 from .postprocessing_template_store import PostprocessingTemplateStore
 
-__all__ = ["DeviceStore", "PictureSourceStore", "PostprocessingTemplateStore"]
+__all__ = ["DeviceStore", "PatternTemplateStore", "PictureSourceStore", "PostprocessingTemplateStore"]