fix: show seekbar on embedded YouTube players

[niveshdandyan]

Summary

Fixes seekbar visibility on embedded YouTube players (e.g., Discogs YouTube Player).

The seekbar, progress bar, video duration, and other player controls were not appearing on embedded YouTube players because the CSS rule that hides controls on small-mode players (used for YouTube's native miniplayer) was also being applied to embedded players on third-party sites.

Fixes #3594

Changes

• Modified CSS selectors in player.css to exclude embedded players (detected via it-pathname starting with /embed) from the control-hiding rules for ytp-small-mode players
• Added 'embed' as a recognized page type in the pageType() function in functions.js for better embedded player detection and potential future enhancements

Root Cause Analysis

The issue was in js&css/extension/www.youtube.com/appearance/player/player.css lines 43-57. The CSS rule:

.html5-video-player.ytp-small-mode .ytp-chrome-bottom {
    opacity: 0 !important;
    pointer-events: none !important;
}

was hiding the entire .ytp-chrome-bottom element (which contains the seekbar, progress bar, and controls) for all players with the ytp-small-mode class. Embedded YouTube players on third-party sites often have this class when displayed in smaller sizes.

Technical Details

• The fix uses the it-pathname attribute (set by the extension from location.pathname) to detect embedded players
• For embedded videos, the pathname is /embed/VIDEO_ID, so we use html:not([it-pathname^="/embed"]) to exclude them from the hiding rule
• The fix preserves the original behavior for YouTube's native miniplayer and minimized players

Testing

• Seekbar appears on embedded YouTube players (pathname starting with /embed/)
• Regular YouTube videos still work correctly
• Native miniplayer on YouTube still hides controls as intended
• No regressions in other features

AI Transparency

This PR was created with the assistance of AI (Claude by Anthropic) for code generation and review.