fix(client): add anti-echo guard for video syncing

[AbdulRashidReshamwala]

Added an anti-echo guard to prevent play/pause event spams.

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Note

Medium Risk
Touches client-side video play/pause state synchronization; a small logic change could still cause playback state to get stuck or miss updates on some browsers (notably Safari).

Overview
Prevents play/pause “echo” loops between the Vuex playing watcher and the <video> element by introducing an isVideoSyncing guard.

When native playing/pause events fire, the component temporarily marks itself as syncing and defers clearing the flag to the next tick, avoiding Safari timing races where events can precede video.paused updates.

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[rgarcia]

thanks @AbdulRashidReshamwala!