refactor: organize frontend by feature

Move the React application into app, api, feature, component, style, and type boundaries, and rebuild the interview scheduler with a responsive details-and-team layout.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

frontend/src/components/ui/AsyncState.tsx

@@ -0,0 +1,17 @@
+export function ErrorAlert({ error }: { error: string }) {
+  return (
+    <div className="error-alert" role="alert">
+      <strong>Request failed</strong>
+      <span>{error}</span>
+    </div>
+  )
+}
+
+export function Loading() {
+  return (
+    <div className="empty-state" role="status">
+      <span className="loading-spinner" />
+      <p>Loading…</p>
+    </div>
+  )
+}