pathways: chevrons only for free directions, fade while previewing, re-render nav on placement

- chevron is only drawn when the direction is not already occupied, so it
  never overlaps a neighboring photo
- hovering a free direction adds a .preview class to the zone so the chevron
  fades out while the ghost preview shows underneath it
- pointermove on an occupied direction early-returns (no ghost, no fade)
- refreshAllNav re-renders every tile's nav after a placement so stale chevrons
  disappear as soon as a direction becomes occupied

1 files changed, 3 insertions, 0 deletions

diff --git a/public/app.css b/public/app.css
index ea00f29..24ea492 100644
--- a/public/app.css
+++ b/public/app.css
@@ -95,6 +95,9 @@ html, body {
   color: #fff;
   filter: drop-shadow(0 0 9px rgba(125, 211, 252, .6));
 }
+.nav-zone.preview .nav-chev {
+  opacity: 0;
+}
 
 #splash {
   position: fixed;