fix: login loading overlay, deduplicate getCookieDomain (F-18, F-11)

- Login page shows LoadingOverlay during redirect instead of blank screen
- Extract getCookieDomain() to shared lib/utils/cookies.ts

app/actions/auth.ts

@@ -2,19 +2,10 @@
 
 import { cookies } from 'next/headers'
 import { logger } from '@/lib/utils/logger'
+import { getCookieDomain } from '@/lib/utils/cookies'
 
 const AUTH_API_URL = process.env.NEXT_PUBLIC_AUTH_API_URL || process.env.NEXT_PUBLIC_AUTH_URL || 'http://localhost:8081'
 
-// * Determine cookie domain dynamically
-// * In production (on ciphera.net), we want to share cookies with subdomains (e.g. pulse-api.ciphera.net)
-// * In local dev (localhost), we don't set a domain
-const getCookieDomain = () => {
-  if (process.env.NODE_ENV === 'production') {
-    return '.ciphera.net'
-  }
-  return undefined
-}
-
 interface AuthResponse {
   access_token: string
   refresh_token: string