fix(auth): expose register_enabled in /api/status and gate sign-up link (#4871)

/api/status never returned `register_enabled` or `password_register_enabled`,
so the sign-in page had no way to react when an admin disabled registration.
The "Sign up" link was only gated on `self_use_mode_enabled`, which is a
separate and unrelated concept (single-user vs. multi-user deployment).

Result: toggling "Registration Enabled" in admin settings had no visible
effect on the login page — users could still see the sign-up link even when
registration was disabled, and could not see it even when it was enabled
(if the system happened to be in self-use mode from initial setup).

Fix:
- Add `register_enabled` and `password_register_enabled` to GetStatus()
- Gate the "Sign up" link on `register_enabled !== false` in addition to
  the existing `!self_use_mode_enabled` check

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

controller/misc.go

@@ -87,6 +87,8 @@ func GetStatus(c *gin.Context) {
 		"chats":                         setting.Chats,
 		"demo_site_enabled":             operation_setting.DemoSiteEnabled,
 		"self_use_mode_enabled":         operation_setting.SelfUseModeEnabled,
+		"register_enabled":              common.RegisterEnabled,
+		"password_register_enabled":     common.PasswordRegisterEnabled,
 		"default_use_auto_group":        setting.DefaultUseAutoGroup,
 
 		"usd_exchange_rate": operation_setting.USDExchangeRate,

web/default/src/features/auth/sign-in/index.tsx

@@ -35,7 +35,7 @@ export function SignIn() {
           <h2 className='text-center text-2xl font-semibold tracking-tight sm:text-left'>
             {t('Sign in')}
           </h2>
-          {!status?.self_use_mode_enabled && (
+          {!status?.self_use_mode_enabled && status?.register_enabled !== false && (
             <p className='text-muted-foreground text-left text-sm sm:text-base'>
               {t("Don't have an account?")}{' '}
               <Link