fix(photoswipe): keyboard arrow animation via CSS keyframes

Previous approach (CSS transition + reflow trick) is unreliable in
Firefox. New approach: PhotoSwipe emits 'change' synchronously before
painting; we add a direction-aware CSS keyframe animation to the
incoming slide element, with animation-fill-mode:both so there is no
flash before the animation starts.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Vgmzx8VTTTmCskSpQtsLTr

themes/intotheeast/css/style.css

@@ -459,6 +459,19 @@ body::after {
 /* iOS Safari: 100vh freezes when address bar hides; dvh tracks the live viewport */
 .pswp { height: 100dvh; }
 
+/* Keyboard arrow navigation slide-in animations */
+.pswp-key-from-right { animation: pswpKeyFromRight 0.35s cubic-bezier(0.4, 0, 0.22, 1) both; }
+.pswp-key-from-left  { animation: pswpKeyFromLeft  0.35s cubic-bezier(0.4, 0, 0.22, 1) both; }
+
+@keyframes pswpKeyFromRight {
+    from { transform: translateX(48px); opacity: 0; }
+    to   { transform: translateX(0);    opacity: 1; }
+}
+@keyframes pswpKeyFromLeft {
+    from { transform: translateX(-48px); opacity: 0; }
+    to   { transform: translateX(0);     opacity: 1; }
+}
+
 /* ── Map page ────────────────────────────────────────────────────────────────── */
 
 .map-page .site-main { max-width: none; padding: 0; }