feat: add page fade transition and CSS performance improvements

- Wrap {#key} children in fade transition (out 100ms, in 180ms+60ms delay)
  for a smooth cross-fade between pages with no added dependencies
- Halve navigation progress bar animation duration (8s → 4s) for
  more realistic feedback on typical navigations
- Add prefers-reduced-motion media query to collapse all animation/transition
  durations for users with accessibility needs
- Add content-visibility: auto on footer to skip browser paint of
  off-screen content and improve rendering performance

ui/src/app.css

@@ -247,5 +247,22 @@ html {
 	100% { width: 100%; opacity: 0; }
 }
 .animate-progress-bar {
-	animation: progress-bar 8s cubic-bezier(0.1, 0.05, 0.1, 1) forwards;
+	animation: progress-bar 4s cubic-bezier(0.1, 0.05, 0.1, 1) forwards;
+}
+
+/* ── Respect reduced motion — disable all decorative animations ─────── */
+@media (prefers-reduced-motion: reduce) {
+	*,
+	*::before,
+	*::after {
+		animation-duration: 0.01ms !important;
+		animation-iteration-count: 1 !important;
+		transition-duration: 0.01ms !important;
+	}
+}
+
+/* ── Footer content-visibility — skip paint for off-screen footer ───── */
+footer {
+	content-visibility: auto;
+	contain-intrinsic-size: auto 80px;
 }