fix: respect device screen timeout; keep screen on only during playback

[chillpill244]

Summary

• MainActivity was calling window.addFlags(FLAG_KEEP_SCREEN_ON) unconditionally at app launch and never clearing it, so the device screen timeout was completely ignored app-wide
• Introduced a KeepScreenOn(active: Boolean) composable that uses DisposableEffect to add/clear the window flag reactively
• Applied it to every playback surface so the screen stays on exactly when something is playing and not otherwise

Affected screens

Screen	Condition
PlayerScreen	Always (screen is a full video player)
TvScreen	playingChannelId != null
LiveTvScreen	playingChannelId != null
DetailsScreen	showTrailerPlayer == true

PiP is also covered — PlayerScreen stays in composition during PiP, so the flag remains set while video plays in the PiP window.

Test plan

• Open the app and leave it idle on Home/Search/Details — screen should time out per device setting
• Start playback in PlayerScreen — screen should stay on indefinitely
• Enter/exit PiP — screen should stay on while PiP video is playing
• Open a live TV channel in TvScreen / LiveTvScreen — screen stays on; navigating away releases the flag
• Open a trailer on a Details screen — screen stays on while trailer is open, times out after closing it

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