[Feature] Background download mode & per‑video notification toggle

[23f2000673]

Describe the feature

1. Background download mode

   Allow the download process to run in the background, so users can continue browsing or performing other tasks without keeping the download dialog open.

2. Per‑video notification toggle

   Add a setting to disable or batch notifications for each video as it completes.

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Mockup (optional) In the Settings panel, under Downloads:

[ ] Enable background downloads
[ ] Show individual video notifications   (when unchecked, only a final “Playlist complete” alert is shown)