Per-channel video limit support for videos widget

The videos widget currently applies a global limit across all channels, which often results in high-frequency channels (e.g. educational or news-focused ones like Khan Academy or BBC) flooding the widget. This unintentionally hides videos from lower-frequency but equally valuable channels. Users who follow multiple channels with different publishing behaviors cannot control content balance effectively.

Introducing per-channel limit support would solve this imbalance. It would give users the ability to display, for example, only 2 videos from frequently-updating channels and 5 from more curated ones — maintaining a clean and intentional dashboard view. Many feed readers (Netvibes, Inoreader, FreshRSS) allow such per-feed control, and having a similar mechanism in glance would bring a significant customization and usability improvement.