Task 7: Build DashboardLayout and wire up App.tsx

Three-zone layout: TopBar (fixed) + Sidebar (fixed left) + Main
(scrollable card grid). Framer Motion staggered entrance animations
with prefers-reduced-motion support. Card grid responsive at 900px.
Replaces PMRInterface in the pmr phase.

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

src/index.css

@@ -265,3 +265,14 @@ html {
 .pmr-scrollbar::-webkit-scrollbar-thumb:hover {
   background: var(--text-tertiary);
 }
+
+/* Dashboard card grid responsive */
+.dashboard-grid {
+  grid-template-columns: repeat(2, 1fr);
+}
+
+@media (max-width: 900px) {
+  .dashboard-grid {
+    grid-template-columns: 1fr;
+  }
+}