Previews appear as you scroll: current tile follows pan, tappable destination previews

- Current tile follows the viewport while dragging (touch) or scrolling
  (wheel), throttled ~200ms, snapping to the nearest visible tile on
  gesture end. setCurrent(id, rect, recenter) so follow never fights the
  user's pan (no recenter, no zoom change).
- Persistent destination previews: the current tile's free exits now
  render the destination photo (ghost) at each spot, always visible on
  mobile and desktop, replacing the old all-tile invisible dropzones and
  the hover-only dropzone ghost. Tapping/clicking a preview walks there.
- Desktop hover ghost kept for non-current tiles only (the current tile's
  previews are already persistent, so no duplicate).
- Dropzones/previews are current-tile-only; ghost preview on nav zones of
  the current tile suppressed.
- Removed MAX_ZONE_TILES (unused after current-only previews).

Tests: domtest 49/49 (rewritten ghost checks for persistent previews +
follow-scroll), touchtest 19/19, exhaust 9/9, splash 6/6, fuzz clean,
hoverlive against live data: 0 stray ghosts, 31 previews across 13 tiles.

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