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/routes/picture_sources.py

@@ -365,26 +365,33 @@ async def test_picture_source(
             )
             stream.initialize()
 
-            logger.info(f"Starting {test_request.capture_duration}s stream test for {stream_id}")
-
             frame_count = 0
             total_capture_time = 0.0
             last_frame = None
-
             start_time = time.perf_counter()
-            end_time = start_time + test_request.capture_duration
 
-            while time.perf_counter() < end_time:
+            if test_request.capture_duration == 0:
+                # Single frame capture
+                logger.info(f"Capturing single frame for {stream_id}")
                 capture_start = time.perf_counter()
                 screen_capture = stream.capture_frame()
                 capture_elapsed = time.perf_counter() - capture_start
-
-                if screen_capture is None:
-                    continue
-
-                total_capture_time += capture_elapsed
-                frame_count += 1
-                last_frame = screen_capture
+                if screen_capture is not None:
+                    total_capture_time = capture_elapsed
+                    frame_count = 1
+                    last_frame = screen_capture
+            else:
+                logger.info(f"Starting {test_request.capture_duration}s stream test for {stream_id}")
+                end_time = start_time + test_request.capture_duration
+                while time.perf_counter() < end_time:
+                    capture_start = time.perf_counter()
+                    screen_capture = stream.capture_frame()
+                    capture_elapsed = time.perf_counter() - capture_start
+                    if screen_capture is None:
+                        continue
+                    total_capture_time += capture_elapsed
+                    frame_count += 1
+                    last_frame = screen_capture
 
             actual_duration = time.perf_counter() - start_time