Add video picture source: file, URL, YouTube, sync clock, trim, test preview

Backend:
- VideoCaptureSource dataclass with url, loop, playback_speed, start/end_time,
  resolution_limit, clock_id, target_fps fields
- VideoCaptureStream: OpenCV decode thread with frame-accurate sync clock seeking,
  loop, trim range, resolution downscale at decode time
- YouTube URL resolution via yt-dlp (auto-detects youtube.com, youtu.be, shorts)
- Thumbnail extraction from first frame (GET /picture-sources/{id}/thumbnail)
- Video test WS preview: streams JPEG frames with elapsed/frame_count metadata
- Run video_stream.start() in executor to avoid blocking event loop during
  yt-dlp resolution
- Full CRUD via existing picture source API (stream_type: "video")
- Wired into LiveStreamManager for target streaming

Frontend:
- Video icon (film) in picture source type map and graph node subtypes
- Video tree nav node in Sources tab with CardSection
- Video fields in stream add/edit modal: URL, loop toggle, playback speed slider,
  target FPS, start/end trim times, resolution limit
- Video card rendering with URL, FPS, loop, speed badges
- Clone data support for video sources
- i18n keys for video source in en/ru/zh

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

server/src/wled_controller/api/routes/_test_helpers.py

@@ -43,6 +43,19 @@ def _encode_jpeg(pil_image: Image.Image, quality: int = 85) -> str:
     return f"data:image/jpeg;base64,{b64}"
 
 
+def encode_preview_frame(image: np.ndarray, max_width: int = None, quality: int = 80) -> bytes:
+    """Encode a numpy RGB image to JPEG bytes, optionally downscaling."""
+    import cv2
+    if max_width and image.shape[1] > max_width:
+        scale = max_width / image.shape[1]
+        new_h = int(image.shape[0] * scale)
+        image = cv2.resize(image, (max_width, new_h), interpolation=cv2.INTER_AREA)
+    # RGB → BGR for OpenCV JPEG encoding
+    bgr = cv2.cvtColor(image, cv2.COLOR_RGB2BGR)
+    _, buf = cv2.imencode('.jpg', bgr, [cv2.IMWRITE_JPEG_QUALITY, quality])
+    return buf.tobytes()
+
+
 def _make_thumbnail(pil_image: Image.Image, max_width: int) -> Image.Image:
     """Create a thumbnail copy of the image, preserving aspect ratio."""
     thumb = pil_image.copy()