feat(frontend): add Dashboard, Backups, and Settings pages

- Create Dashboard page with stats cards and recent activity feed
- Create Backups page with job listing table, create modal, and actions
- Create Settings page with tabs for General, Notifications, Security, Logs
- Update App.tsx to use new page components
- Add delete method to jobsApi in client.ts
- TypeScript compiles cleanly

frontend/src/api/client.ts

@@ -51,6 +51,7 @@ export const jobsApi = {
   getById: (id: string) => apiClient.get<BackupJob>(`/jobs/${id}`),
   create: (sourceId: string) =>
     apiClient.post<BackupJob>('/jobs', { source_id: sourceId }),
+  delete: (id: string) => apiClient.delete(`/jobs/${id}`),
   getLogs: (id: string) => apiClient.get<string>(`/jobs/${id}/logs`),
 };