Implement UI improvements and download management features

- Switch to light mode with black and white color scheme
- Simplify channel subscription to use channel ID only instead of RSS URL
- Add Downloads page to track all video download jobs
- Fix Flask-Login session management bug in user loader
- Always filter YouTube Shorts from feeds (case-insensitive)
- Fix download service video URL attribute error
- Fix watch page enum comparison for download status display

UI Changes:
- Update CSS to pure black/white/grayscale theme
- Remove colored text and buttons
- Use underlines for hover states instead of color changes
- Improve visual hierarchy with grayscale shades

Channel Subscription:
- Accept channel ID directly instead of full RSS URL
- Add validation for channel ID format (UC/UU prefix)
- Update help text and examples for easier onboarding

Downloads Page:
- New route at /downloads showing all video download jobs
- Display status, progress, and metadata for each download
- Sortable by status (downloading, pending, failed, completed)
- Actions to download, retry, or watch videos
- Responsive grid layout with thumbnails

Bug Fixes:
- Fix user loader to properly use database session context manager
- Fix download service accessing wrong attribute (link → video_url)
- Fix watch page template enum value comparisons
- Fix session detachment issues when accessing channel data

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Dockerfile

@@ -9,7 +9,7 @@ RUN apt-get update && apt-get install -y \
 
 # Install uv for faster Python package management
 RUN curl -LsSf https://astral.sh/uv/install.sh | sh
-ENV PATH="/root/.cargo/bin:$PATH"
+ENV PATH="/root/.local/bin:$PATH"
 
 # Set working directory
 WORKDIR /app
@@ -30,4 +30,4 @@ RUN mkdir -p downloads
 EXPOSE 5000
 
 # Default command (can be overridden in docker-compose)
-CMD ["flask", "--app", "main", "run", "--host=0.0.0.0"]
+CMD ["uv", "run", "flask", "--app", "main", "run", "--host=0.0.0.0"]