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/pattern_template.py

@@ -0,0 +1,47 @@
+"""Pattern template data model for key color rectangle layouts."""
+
+from dataclasses import dataclass, field
+from datetime import datetime
+from typing import List, Optional
+
+from wled_controller.storage.key_colors_picture_target import KeyColorRectangle
+
+
+@dataclass
+class PatternTemplate:
+    """Pattern template containing a named layout of key color rectangles."""
+
+    id: str
+    name: str
+    rectangles: List[KeyColorRectangle]
+    created_at: datetime
+    updated_at: datetime
+    description: Optional[str] = None
+
+    def to_dict(self) -> dict:
+        """Convert to dictionary."""
+        return {
+            "id": self.id,
+            "name": self.name,
+            "rectangles": [r.to_dict() for r in self.rectangles],
+            "created_at": self.created_at.isoformat(),
+            "updated_at": self.updated_at.isoformat(),
+            "description": self.description,
+        }
+
+    @classmethod
+    def from_dict(cls, data: dict) -> "PatternTemplate":
+        """Create from dictionary."""
+        rectangles = [KeyColorRectangle.from_dict(r) for r in data.get("rectangles", [])]
+        return cls(
+            id=data["id"],
+            name=data["name"],
+            rectangles=rectangles,
+            created_at=datetime.fromisoformat(data["created_at"])
+            if isinstance(data.get("created_at"), str)
+            else data.get("created_at", datetime.utcnow()),
+            updated_at=datetime.fromisoformat(data["updated_at"])
+            if isinstance(data.get("updated_at"), str)
+            else data.get("updated_at", datetime.utcnow()),
+            description=data.get("description"),
+        )