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/api/auth/refresh/route.ts

@@ -1,16 +1,9 @@
 import { cookies } from 'next/headers'
 import { NextResponse } from 'next/server'
+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
-const getCookieDomain = () => {
-  if (process.env.NODE_ENV === 'production') {
-    return '.ciphera.net'
-  }
-  return undefined
-}
-
 export async function POST() {
   const cookieStore = await cookies()
   const refreshToken = cookieStore.get('refresh_token')?.value