feat: add full-screen back-to-YouTube button (#3137)

[suryaprakash0010]

Problem

Users who run videos in fullscreen by default cannot easily return to YouTube.
Currently, they must exit fullscreen first to access the YouTube homepage or subscriptions.

Solution

This PR implements a new "Return to YouTube" button in the fullscreen player.

• Positioned in the top-left corner (instead of the video title area).
• Uses official YouTube logo SVG for familiarity.
• Navigates to:
  • YouTube homepage (/), or
  • Subscriptions feed (/feed/subscriptions)
    based on user preference.
• Appears only in fullscreen mode.

Features

✅ Added ImprovedTube.addYouTubeReturnButton() in player.js
✅ Added CSS in player.css for styling & positioning
✅ Integrated MutationObserver to detect fullscreen state changes
✅ Added setting options:
- player_youtube_return_button (toggle on/off)
- player_youtube_return_destination (homepage / subscriptions)
✅ Proper initialization in functions.js

Closes

Closes #3137

[ImprovedTube]

thank you so much! @suryaprakash0010

[ImprovedTube]

lets add a clear descriptive text to locales/en/messages.json

player_youtube_return_destination could also enhance our feature "auto-full screen" listed above it.
to both features it is a sub-option, we can show conditionally only when the feature is enabled:
https://github.com/code-charity/youtube/blob/master/menu/styles/sub-options.css

[ImprovedTube]

hi, using history.back(); so that the page doesn't reload

and removed:
if (ImprovedTube.storage.player_youtube_return_destination === 'subscriptions') {
window.location.href = '/feed/subscriptions';
} else {
window.location.href = '/';
}

but we can add more buttons, such as:

	fullscreen_link_button: {
		component: 'select',
		text: 'destination',
		options: [{
			text: 'none',
			value: 'none'
		}, {
			text: 'URL',
			value: 'url'  
			
			//to add an input: check appearance > player > size > custom.

		}{
			text: 'homepage',
			value: 'homepage'
		}, {
			text: 'subscriptions',
			value: 'subscriptions'
		}],
		value: 'none'
	},	

[ImprovedTube]

( and we can enhance this feature to go history.back() when closing fullscreen as well)

/------------------------------------------------------------------------------
AUTO FULLSCREEN
------------------------------------------------------------------------------/
ImprovedTube.playerAutofullscreen = function () {
if (
this.storage.player_autofullscreen === true &&
document.documentElement.dataset.pageType === 'video' &&
!document.fullscreenElement
) {
this.elements.player.toggleFullscreen();
}
};