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/templates/modals/stream.html

@@ -92,6 +92,52 @@
                     <div id="stream-image-validation-status" class="validation-status" style="display: none;"></div>
                 </div>
 
+                <div id="stream-video-fields" style="display: none;">
+                    <div class="form-group">
+                        <div class="label-row">
+                            <label for="stream-video-url" data-i18n="picture_source.video.url">Video URL:</label>
+                            <button type="button" class="hint-toggle" onclick="toggleHint(this)" title="?">?</button>
+                        </div>
+                        <small class="input-hint" style="display:none" data-i18n="picture_source.video.url.hint">Local file path, HTTP URL, or YouTube URL</small>
+                        <input type="text" id="stream-video-url" data-i18n-placeholder="picture_source.video.url.placeholder" placeholder="https://example.com/video.mp4">
+                    </div>
+                    <div class="form-group settings-toggle-group">
+                        <label data-i18n="picture_source.video.loop">Loop:</label>
+                        <label class="settings-toggle">
+                            <input type="checkbox" id="stream-video-loop" checked>
+                            <span class="settings-toggle-slider"></span>
+                        </label>
+                    </div>
+                    <div class="form-row">
+                        <div class="form-group" style="flex:1">
+                            <label for="stream-video-speed" data-i18n="picture_source.video.speed">Playback Speed: <span id="stream-video-speed-value">1.0</span>×</label>
+                            <input type="range" id="stream-video-speed" min="0.1" max="5" step="0.1" value="1.0" oninput="document.getElementById('stream-video-speed-value').textContent=this.value">
+                        </div>
+                        <div class="form-group" style="flex:1">
+                            <label for="stream-video-fps" data-i18n="streams.target_fps">Target FPS:</label>
+                            <input type="number" id="stream-video-fps" min="1" max="60" step="1" value="30">
+                        </div>
+                    </div>
+                    <div class="form-row">
+                        <div class="form-group" style="flex:1">
+                            <label for="stream-video-start" data-i18n="picture_source.video.start_time">Start Time (s):</label>
+                            <input type="number" id="stream-video-start" min="0" step="0.1">
+                        </div>
+                        <div class="form-group" style="flex:1">
+                            <label for="stream-video-end" data-i18n="picture_source.video.end_time">End Time (s):</label>
+                            <input type="number" id="stream-video-end" min="0" step="0.1">
+                        </div>
+                    </div>
+                    <div class="form-group">
+                        <div class="label-row">
+                            <label for="stream-video-resolution" data-i18n="picture_source.video.resolution_limit">Max Width (px):</label>
+                            <button type="button" class="hint-toggle" onclick="toggleHint(this)" title="?">?</button>
+                        </div>
+                        <small class="input-hint" style="display:none" data-i18n="picture_source.video.resolution_limit.hint">Downscale video at decode time for performance</small>
+                        <input type="number" id="stream-video-resolution" min="64" max="7680" step="1" placeholder="720">
+                    </div>
+                </div>
+
                 <div class="form-group">
                     <label for="stream-description" data-i18n="streams.description_label">Description (optional):</label>
                     <input type="text" id="stream-description" data-i18n-placeholder="streams.description_placeholder" placeholder="Describe this source...">