[players] Tighten panzoom and add scrub/focus shortcuts

[frankrousseau]

Problems

• Panzoom bound against 0×0 / gutter-sized media: off-centre on slow loads,
  triggered by clicks in the area around the media, and stole the player's
  keyboard shortcuts.
• Annotation overlay spilled across PlaylistPlayer comparison viewers.
• PlaylistPlayer comparison opened with panzoom off-centre.
• PreviewPlayer fullscreen ignored browser zoom — black bar at <100 %
  (Preview Player on Main Page doesn't use browser scaling for the videoplayer #1541).
• No keyboard scrub / focus toggle between player and comment textarea.
• Pen / touch drag along the timeline triggered Chrome's back-forward
  swipe and dropped unsaved comments (Dragging on timeline with touch / pen causes Chrome to navigate backward or forward #1700).

Solutions

• Gate panzoom binding on the media's natural size, add
  beforeMouseDown / beforeWheel guards, and set
  disableKeyboardInteraction: true.
• Wrap each annotation canvas in a per-viewer slot clipped at the
  viewer's edge.
• Defer MultiVideoViewer panzoom to loadedmetadata and reset it once
  bound.
• Read defaultHeight from reactive window.innerWidth / innerHeight.
• Shift+drag scrubs from anywhere in the player; Shift+Tab toggles focus
  with the comment textarea; nested players let the innermost handle the
  event.
• overscroll-behavior-x: contain on player + timeline, touch-action: pan-y on progress bars.