feat: add AppShell + Topbar + 6-route AppRouter with stub pages

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

frontend/src/components/layout/Topbar.tsx

@@ -0,0 +1,17 @@
+interface TopbarProps {
+  title: string;
+  subtitle?: string;
+  actions?: React.ReactNode;
+}
+
+export function Topbar({ title, subtitle, actions }: TopbarProps) {
+  return (
+    <header className="topbar">
+      <div className="topbar-left">
+        <h1 className="topbar-title">{title}</h1>
+        {subtitle && <span className="topbar-sub">{subtitle}</span>}
+      </div>
+      {actions && <div className="topbar-actions">{actions}</div>}
+    </header>
+  );
+}