feat: include channel name in screenshot filename

[Gilmoursa]

Summary

Closes #3854.

• Reads the channel name from the page using the .ytd-channel-name a selector (with #upload-info .ytd-channel-name as a fallback) — the same selectors already used elsewhere in player.js
• Appends the channel name between the timestamp and video title in the screenshot filename
• Falls back gracefully (channel name simply omitted) if the element is not found on the page

Before: {videoId} {timestamp} {videoTitle}.png
After: {videoId} {timestamp} {channelName} {videoTitle}.png

[ImprovedTube2]

thank you @Gilmoursa