Left/right on the Siri Remote now seek instead of revealing controls,
reusing the iOS tap-seek accumulation handler and feedback overlay so
rapid presses compound into a single "+30s" / "-20s" jump. Up/down
still show controls.
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().
Extract chapter title from inside the storyboard preview into a
standalone ChapterCapsuleView with its own glass capsule background.
The capsule follows the seek position horizontally but independently
clamps to screen edges using alignmentGuide, allowing it to be wider
than the storyboard thumbnail without going offscreen.
