Refactor backend architecture and update documentation

- Add CSRF protection middleware implementation
- Update API client with improved configuration
- Enhance documentation for backend development
- Add architecture documentation updates
- Reorganize and clean up task documentation

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

frontend/src/api/client.ts

@@ -86,6 +86,7 @@ async function request<T>(url: string, options: RequestInit = {}): Promise<T> {
     credentials: "include",
     headers: {
       "Content-Type": "application/json",
+      "X-BanGUI-Request": "1",
       ...(options.headers as Record<string, string> | undefined),
     },
   });