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.

1 files changed, 12 insertions, 1 deletions

diff --git a/public/app.css b/public/app.css
index 6d5e992..5640b1c 100644
--- a/public/app.css
+++ b/public/app.css
@@ -49,7 +49,18 @@ html, body {
   pointer-events: none;
   box-shadow: 0 0 0 2px rgba(125, 211, 252, .65);
 }
-.dropzone { position: absolute; }
+.dropzone { position: absolute; cursor: pointer; }
+.dropzone .preview {
+  width: 100%;
+  height: 100%;
+  opacity: .5;
+  pointer-events: none;
+  border-radius: 6px;
+  box-shadow: 0 0 0 2px rgba(125, 211, 252, .5);
+  transition: opacity .15s;
+}
+.dropzone:hover .preview,
+.dropzone:active .preview { opacity: .85; }
 
 @keyframes lock {
   0%   { transform: scale(.2); opacity: 0; }