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.

app/login/actions.ts

@@ -3,6 +3,7 @@
 import { revalidatePath } from 'next/cache'
 import { redirect } from 'next/navigation'
 import { createClient } from '@/lib/supabase/server'
+import { getFirstAdminSetupState } from '@/lib/auth/first-admin-setup'
 
 export async function login(formData: FormData) {
   const supabase = await createClient()
@@ -23,6 +24,20 @@ export async function login(formData: FormData) {
 }
 
 export async function signup(formData: FormData) {
+  const setupState = await getFirstAdminSetupState()
+
+  if (setupState.errorMessage) {
+    redirect(`/register?error=true&message=${encodeURIComponent(setupState.errorMessage)}`)
+  }
+
+  if (!setupState.available) {
+    redirect(
+      `/login?error=true&message=${encodeURIComponent(
+        'Kayıt kapalı. Bu self-host kurulumunda ilk admin hesabı zaten oluşturulmuş.',
+      )}`,
+    )
+  }
+
   const supabase = await createClient()
 
   const data = {
@@ -33,7 +48,7 @@ export async function signup(formData: FormData) {
   const { error } = await supabase.auth.signUp(data)
 
   if (error) {
-    redirect(`/login?error=true&message=${encodeURIComponent(error.message)}`)
+    redirect(`/register?error=true&message=${encodeURIComponent(error.message)}`)
   }
 
   revalidatePath('/', 'layout')