feat(app): base-aware routing, embedded flag, panel slide animation

- Use import.meta.env.BASE_URL for router history (Archy /aiui/ deployments)
- Capture embedded flag from URL params before router init
- Add panelSlideIn animation with prefers-reduced-motion support

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

packages/app/src/styles/main.css

@@ -605,6 +605,27 @@ input:focus-visible {
   50% { opacity: 1; }
 }
 
+@keyframes panelSlideIn {
+  0% {
+    opacity: 0;
+    transform: translateX(-40px) scale(0.97);
+  }
+  100% {
+    opacity: 1;
+    transform: translateX(0) scale(1);
+  }
+}
+
+.panel-slide-in {
+  animation: panelSlideIn 0.6s cubic-bezier(0.22, 1, 0.36, 1) both;
+}
+
+@media (prefers-reduced-motion: reduce) {
+  .panel-slide-in {
+    animation: none;
+  }
+}
+
 /* ===== POSTER CARD — path-glass border ===== */
 
 .poster-card {