feat: add youtube playlist support

[Ranjesh2002]

Summary

Adds YouTube playlist support (fixes #9).

Previously, pasting a playlist URL would only fetch the first video because --no-playlist was passed to yt-dlp in both the info and download commands. This PR removes that restriction and handles playlists end-to-end.

Changes

app.py

• Removed --no-playlist from get_info() and run_download()

• Updated get_info() to parse multiple newline-delimited JSON objects returned by yt-dlp for playlists

• Returns { is_playlist: true, videos: [...] } when multiple videos are detected, single video response unchanged for
  backward compatibility

  templates/index.html

• Updated go() to detect is_playlist response and expand each video into its own card

• Playlist videos behave identically to manually pasted individual URLs — individual download + Download All both work

  Testing

• Tested with a 13-video YouTube playlist

• Single video URLs still work as before

• "Download All" works across playlist videos

Handles both playlist URL formats:

`https://www.youtube.com/playlist?list=xxx
`https://www.youtube.com/watch?v=xxx&list=xxx

playlist-download.mp4

[AmanoSpica]

Thanks for the work on this!

I was testing playlist handling and noticed that using /api/info for playlists can be a bit slow due to how yt-dlp fetches the data.

I tried a slightly different approach in my PR (#28), where the playlist is expanded into individual video URLs beforehand using yt-dlp --flat-playlist.

With this approach:

• Each video is processed independently
• Results can be shown progressively
• It feels a bit more responsive from a UX perspective

Not sure if this direction fits the project, but I’d appreciate your thoughts!

[Ranjesh2002]

Thanks for the suggestion and for testing! I actually pulled your branch locally and tested it with the same playlist. it only fetched the first video rather than all videos in the playlist, so it doesn't fully resolve issue #9.
My implementation works as follows:

• Removed --no-playlist from both get_info() and run_download()
• Parses the multiple newline-delimited JSON objects yt-dlp - returns for playlists
• All videos in the playlist are rendered as individual cards
• Users can download individually or use "Download All"

Tested with a 13-video playlist all 13 videos fetched successfully. Single video URLs continue to work as before.