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/router.js

@@ -350,20 +350,12 @@ async function renderPage(route, previousPath = null) {
 
     // Richtung bestimmen (previousPath ist der alte Pfad vor der Navigation)
     const direction = getDirection(previousPath, route.path);
-    const outClass  = direction === 'right' ? 'page-transition--out-left' : 'page-transition--out-right';
     const inClass   = direction === 'right' ? 'page-transition--in-right' : 'page-transition--in-left';
 
     // Performance: backdrop-filter während Übergang deaktivieren (Android-Optimierung).
     // glass.css setzt alle backdrop-filter im app-content auf none solange diese Klasse aktiv ist.
     document.documentElement.classList.add('navigating');
 
-    // Alte Seite kurz ausfaden, falls vorhanden
-    const oldPage = content.querySelector('.page-transition');
-    if (oldPage) {
-      oldPage.classList.add(outClass);
-      await new Promise(r => setTimeout(r, 120));
-    }
-
     // Alter Inhalt ist jetzt weg - altes Stylesheet kann entfernt werden
     const pageWrapper = document.createElement('div');
     pageWrapper.className = 'page-transition';