fix: page transition crossfade eliminates dark-mode flash on navigation

Removes the sequential fade-out → wait → fade-in pattern that causes
a visible black flash in dark mode between page transitions. Replaces
with immediate crossfade (new page fades in over old content, no wait).

Changes:
- layout.css: Add page-crossfade-in keyframe (0.18s) + prefers-reduced-motion override
- router.js: Remove outClass/inClass direction logic and oldPage fadeout wait

The new approach:
1. Old page remains visible until new page renders
2. New page fades in (0.18s) with full opacity, overlaying old content
3. No 120ms delay = no visible flash in dark mode

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

public/styles/layout.css

@@ -80,6 +80,11 @@
   to   { opacity: 0; transform: translateX(20px); }
 }
 
+@keyframes page-crossfade-in {
+  from { opacity: 0; }
+  to   { opacity: 1; }
+}
+
 .page-transition--in-right {
   animation: page-slide-in-right 0.2s var(--ease-out);
 }
@@ -95,6 +100,11 @@
   pointer-events: none;
 }
 
+.page-transition--crossfade {
+  animation: page-crossfade-in 0.18s var(--ease-out);
+  opacity: 1;
+}
+
 @media (prefers-reduced-motion: reduce) {
   .page-transition--in-right,
   .page-transition--in-left {
@@ -105,6 +115,10 @@
   .page-transition--out-right {
     animation: none;
   }
+  .page-transition--crossfade {
+    animation: none;
+    opacity: 1;
+  }
 }
 
 /* --------------------------------------------------------