feat: Implement first admin setup guard to lock registration after initial account creation

- Updated the registration flow to check if the first admin account has been created, preventing further public registrations.
- Introduced `is_first_admin_setup_available` function to determine registration availability.
- Modified the `/register` and `/login` pages to redirect based on the setup state.
- Enhanced the user creation process to handle internal admin accounts correctly.
- Added migration script to enforce the new registration rules in the database.
- Refactored chat API to improve message handling and context building.
- Updated dashboard and settings components for better state management.
- Improved error handling and user feedback across various components.

lib/auth/first-admin-setup.ts

@@ -0,0 +1,21 @@
+import { createClient } from "@/lib/supabase/server";
+
+type FirstAdminSetupState = {
+  available: boolean;
+  errorMessage?: string;
+};
+
+export async function getFirstAdminSetupState(): Promise<FirstAdminSetupState> {
+  const supabase = await createClient();
+  const { data, error } = await supabase.rpc("is_first_admin_setup_available");
+
+  if (error) {
+    return {
+      available: false,
+      errorMessage:
+        "İlk kurulum kontrolü yapılamadı. 0009 kayıt kilidi migration dosyasını çalıştırdığından emin ol.",
+    };
+  }
+
+  return { available: Boolean(data) };
+}