Pressing left/right on the focused progress bar now triggers the same
accumulating 10s seek as the hidden-controls flow, but updates the
visible scrubber in place with no overlay. Both tvOS arrow-seek paths
accumulate a signed net offset so a reverse press subtracts from the
pending amount instead of restarting from the current playback time.
Extract shared TimeInterval.formattedAsTimestamp replacing 8 identical
formatTime/formattedTime implementations across player views. Remove
unused currentTime parameter from GestureSeekPreviewView. Consolidate
duplicated geometry math in MacOSControlBar into seekPreviewPosition().
