feat: scaffold frontend app structure

frontend/src/app/layouts/AppLayout.tsx

@@ -0,0 +1,33 @@
+import { NavLink, Outlet } from "react-router-dom";
+
+export function AppLayout(): JSX.Element {
+  return (
+    <div className="app-shell">
+      <header className="app-shell__header">
+        <div className="app-shell__brand">Calendar Run</div>
+        <nav className="app-shell__nav" aria-label="Primary navigation">
+          <NavLink
+            to="/"
+            end
+            className={({ isActive }) =>
+              isActive ? "app-shell__link app-shell__link--active" : "app-shell__link"
+            }
+          >
+            Dashboard
+          </NavLink>
+          <NavLink
+            to="/races"
+            className={({ isActive }) =>
+              isActive ? "app-shell__link app-shell__link--active" : "app-shell__link"
+            }
+          >
+            Races
+          </NavLink>
+        </nav>
+      </header>
+      <main className="app-shell__main">
+        <Outlet />
+      </main>
+    </div>
+  );
+}