feat: YouTube/Instagram/MP3 download with modal + yt-dlp support

Sync from gaming-hub soundboard plugin:
- Add yt-dlp URL detection (YouTube, Instagram) + direct MP3 support
- downloadWithYtDlp() with verbose logging, error detection, fallback scan
- handleUrlDownload() shared logic with custom filename + rename
- Download modal: filename input, progress spinner, success/error phases
- URL type badges (YT/IG/MP3) in toolbar input
- Auto-prepend https:// for URLs without protocol
- Fix Dockerfile: yt-dlp_linux standalone binary (no Python needed)
- download-url route (admin-only, save without playing)

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

Dockerfile

@@ -41,7 +41,7 @@ ENV PORT=8080
 ENV SOUNDS_DIR=/data/sounds
 
 RUN apt-get update && apt-get install -y --no-install-recommends ca-certificates curl \
- && curl -L https://github.com/yt-dlp/yt-dlp/releases/latest/download/yt-dlp -o /usr/local/bin/yt-dlp \
+ && curl -L https://github.com/yt-dlp/yt-dlp/releases/latest/download/yt-dlp_linux -o /usr/local/bin/yt-dlp \
  && chmod a+rx /usr/local/bin/yt-dlp \
  && apt-get purge -y curl && apt-get autoremove -y && rm -rf /var/lib/apt/lists/*