Add high-quality GIF export support

[ax2bboud]

This PR introduces the ability for users to download videos directly as optimized GIFs.

Key Changes

• Frontend (index.html):
  • Added a new GIF toggle button to the format selection group.
  • Ensures the format parameter is correctly passed to the API during the download request.
• Backend (app.py):
  • Implemented a new processing branch for the gif format.
  • Integrated ffmpeg for transcoding with a focus on quality and efficiency:
    • Framerate: Set to 15fps for smooth motion without excessive file size.
    • Resolution: Scaled to a maximum width of 480px (aspect-ratio preserved) to keep downloads fast.
    • Quality: Uses a two-pass filter (palettegen and paletteuse) to ensure the GIF uses an optimized color palette for better visual fidelity.
  • Automated cleanup of the intermediate MP4 file once the GIF conversion is complete.

How to Test

1. Paste a video URL (e.g., from YouTube or TikTok).
2. Click the GIF toggle in the format options.
3. Click Fetch, then click Download.
4. Verify that the resulting file is a high-quality .gif.

[ax2bboud]

Ideally, you'd want people to choose quality and fps options but it would take away from the simplicity of the tool.