feat(01-04): layout, routing, API hooks, tipi TypeScript, Dashboard, Settings

- types.ts: CalendarSlot, GeneratedPost, PostResult, JobStatus, Settings, SettingsStatus
- api/client.ts: aggiunto apiGet, apiPost, apiPut, apiDownload, triggerDownload
- api/hooks.ts: 10+ hooks TanStack Query (settings, generate, job polling, CSV export)
- components/Layout.tsx + Sidebar.tsx: sidebar stone/amber palette con 4 nav link
- pages/Dashboard.tsx: banner API key, quick actions link, step guide
- pages/Settings.tsx: form completo (API key password, LLM select, brand, nicchie checkbox)
- App.tsx: 5 route con BrowserRouter basename='/postgenerator', QueryClientProvider, Layout

frontend/src/pages/GenerateCalendar.tsx

@@ -0,0 +1,6 @@
+/**
+ * Pagina Genera Calendario (stub — completata nel Task 2c)
+ */
+export function GenerateCalendar() {
+  return <div className="p-6 text-stone-400">Genera Calendario — in costruzione</div>
+}