Add Key Colors target type for extracting colors from screen regions

Introduce a new "key_colors" target type alongside WLED targets, enabling
real-time color extraction from configurable screen rectangles with
average/median/dominant modes, temporal smoothing, and WebSocket streaming.

- Split WledPictureTarget into its own module, add KeyColorsPictureTarget
- Add KC target lifecycle to ProcessorManager (register, start/stop, processing loop)
- Extend API routes and schemas for KC targets (CRUD, settings, state, metrics, colors)
- Add WebSocket endpoint for real-time color updates with auth
- Add KC sub-tab in Targets UI with editor modal and live color swatches
- Add EN and RU translations for all key colors strings

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

server/src/wled_controller/static/locales/en.json

@@ -341,5 +341,40 @@
   "targets.metrics.actual_fps": "Actual FPS",
   "targets.metrics.target_fps": "Target FPS",
   "targets.metrics.frames": "Frames",
-  "targets.metrics.errors": "Errors"
+  "targets.metrics.errors": "Errors",
+  "targets.subtab.key_colors": "Key Colors",
+  "targets.section.key_colors": "🎨 Key Colors Targets",
+  "kc.add": "Add Key Colors Target",
+  "kc.edit": "Edit Key Colors Target",
+  "kc.name": "Target Name:",
+  "kc.name.placeholder": "My Key Colors Target",
+  "kc.source": "Picture Source:",
+  "kc.source.hint": "Which picture source to extract colors from",
+  "kc.source.none": "-- No source assigned --",
+  "kc.fps": "Extraction FPS:",
+  "kc.fps.hint": "How many times per second to extract colors (1-60)",
+  "kc.interpolation": "Color Mode:",
+  "kc.interpolation.hint": "How to compute the key color from pixels in each rectangle",
+  "kc.interpolation.average": "Average",
+  "kc.interpolation.median": "Median",
+  "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.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.required": "Please fill in all required fields",
+  "kc.colors.none": "No colors extracted yet"
 }