Fix cursor hidden and audio leak after closing media viewer

[SnoElement]

Fixes #1303

Also fixes #269

Changes

Bug 1 - Cursor stays hidden system-wide after media viewer closes

Root cause: macOS cursor visibility uses a reference-counted system - NSCursor.hide() and NSCursor.unhide() must be balanced. Two separate code paths in SVideoController called NSCursor.hide():

1. updateIdleTimer() - hides the cursor after 1 second of mouse idle
2. updateControlVisibility(true) (fullscreen mouseDown/mouseUp path) - also calls NSCursor.hide() independently

But viewDidDisappear() only called NSCursor.unhide() once, leaving the reference count unbalanced when both paths had fired. Additionally, GalleryViewer.close(animated: false) ordered the window out without ever calling item.disappear(), so viewDidDisappear() (and its NSCursor.unhide()) never ran at all on non-animated closes (for example pressing Escape).

Fixes (SVideoController.swift, GalleryViewer.swift):

• SVideoController: Introduced isCursorHidden: Bool to track cursor state. Every NSCursor.hide() is guarded by !isCursorHidden and every NSCursor.unhide() is guarded by isCursorHidden, guaranteeing exactly one matched pair per lifecycle regardless of which code paths fire.
• GalleryViewer.close(animated:): Call pager.selectedItem?.disappear(for: nil) before window.orderOut(nil) in the non-animated path, ensuring viewDidDisappear always runs on close.

Bug 2 - Audio continues playing after navigating away or closing the viewer

Root cause: In MediaPlayerContext.seekingCompleted(), the current playback action (for example .play) is read from self.state and captured into the audioRenderer.flushBuffers(completion:) closure. While the flush is in flight, if pause() is called (for example the user navigates to the next item or closes the viewer), self.state's action is updated to .pause - but the completion closure uses the stale captured .play, sets state to .playing, and starts the audio renderer anyway.

This is particularly noticeable with large remote videos on slow connections, where buffering causes a long flush that outlasts viewer dismissal.

Fix (packages/TelegramMedia/Sources/MediaPlayer.swift):

• In the flushBuffers completion closure, re-read the current action from self.state (when still in .seeking) rather than using the value captured at seek time, so any pause() call made during the flush is correctly honoured.

Bug 3 - Full-screen media viewer behaves incorrectly across multiple monitors

Root cause: GalleryViewer always created its full-screen window on NSScreen.main and forcibly reclaimed key-window status on resign. When the app focus moved to another display, the viewer could close unexpectedly or leave Telegram in a bad state that required relaunch.

Fixes (GalleryViewer.swift, AppDelegate.swift):

• Open the gallery window on the app window's current screen instead of hardcoding NSScreen.main.
• Stop forcibly re-focusing the gallery window on resign-key.
• When app and viewer screens diverge, close the viewer cleanly instead of leaving stale viewer state behind.

[CLAassistant]

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