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/static/locales/en.json

@@ -360,21 +360,41 @@
   "kc.interpolation.dominant": "Dominant",
   "kc.smoothing": "Smoothing:",
   "kc.smoothing.hint": "Temporal blending between extractions (0=none, 1=full)",
-  "kc.rectangles": "Color Rectangles",
-  "kc.rectangles.hint": "Define named rectangles in relative coordinates (0.0–1.0) on the captured image",
-  "kc.rect.name": "Name",
-  "kc.rect.x": "X",
-  "kc.rect.y": "Y",
-  "kc.rect.width": "W",
-  "kc.rect.height": "H",
-  "kc.rect.add": "Add Rectangle",
-  "kc.rect.remove": "Remove",
-  "kc.rect.empty": "No rectangles defined. Add at least one rectangle to extract colors.",
+  "kc.pattern_template": "Pattern Template:",
+  "kc.pattern_template.hint": "Select the rectangle pattern to use for color extraction",
+  "kc.pattern_template.none": "-- Select a pattern template --",
   "kc.created": "Key colors target created successfully",
   "kc.updated": "Key colors target updated successfully",
   "kc.deleted": "Key colors target deleted successfully",
   "kc.delete.confirm": "Are you sure you want to delete this key colors target?",
-  "kc.error.no_rectangles": "Please add at least one rectangle",
+  "kc.error.no_pattern": "Please select a pattern template",
   "kc.error.required": "Please fill in all required fields",
-  "kc.colors.none": "No colors extracted yet"
+  "kc.colors.none": "No colors extracted yet",
+  "targets.section.pattern_templates": "📄 Pattern Templates",
+  "pattern.add": "📄 Add Pattern Template",
+  "pattern.edit": "📄 Edit Pattern Template",
+  "pattern.name": "Template Name:",
+  "pattern.name.placeholder": "My Pattern Template",
+  "pattern.description_label": "Description (optional):",
+  "pattern.description_placeholder": "Describe this pattern...",
+  "pattern.rectangles": "Rectangles",
+  "pattern.rect.name": "Name",
+  "pattern.rect.x": "X",
+  "pattern.rect.y": "Y",
+  "pattern.rect.width": "W",
+  "pattern.rect.height": "H",
+  "pattern.rect.add": "Add Rectangle",
+  "pattern.rect.remove": "Remove",
+  "pattern.rect.empty": "No rectangles defined. Add at least one rectangle.",
+  "pattern.created": "Pattern template created successfully",
+  "pattern.updated": "Pattern template updated successfully",
+  "pattern.deleted": "Pattern template deleted successfully",
+  "pattern.delete.confirm": "Are you sure you want to delete this pattern template?",
+  "pattern.delete.referenced": "Cannot delete: this template is referenced by a target",
+  "pattern.error.required": "Please fill in all required fields",
+  "pattern.visual_editor": "Visual Editor",
+  "pattern.capture_bg": "Capture Background",
+  "pattern.source_for_bg": "Source for Background:",
+  "pattern.source_for_bg.none": "-- Select source --",
+  "pattern.delete_selected": "Delete Selected"
 }