feat: add app shell with sidebar navigation, routing, and placeholder pages

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

src/components/layout/app-shell.tsx

@@ -0,0 +1,18 @@
+import type { ReactNode } from "react";
+import { useLocation } from "react-router";
+import { Sidebar } from "./sidebar";
+
+interface AppShellProps {
+    children: ReactNode;
+}
+
+export const AppShell = ({ children }: AppShellProps) => {
+    const { pathname } = useLocation();
+
+    return (
+        <div className="flex min-h-screen bg-primary">
+            <Sidebar activeUrl={pathname} />
+            <main className="flex flex-1 flex-col overflow-auto">{children}</main>
+        </div>
+    );
+};