feat(deploy): add DNS verification with modal before proceeding to next step

Add standalone DNS check endpoint and verify DNS on Next click in the
deploy wizard (step 2). Show error modal with CNAME instructions when
DNS is not configured instead of disabling the Next button. Also make
the custom domain clickable in the app detail header.

Co-authored-by: Cursor <cursoragent@cursor.com>

frontend/src/app/dashboard/apps/[id]/page.tsx

@@ -737,7 +737,11 @@ export default function AppDetailPage() {
               </span>
             </div>
             <p className="text-sm text-gray-500 truncate">
-              {app.runtime}{app.runtimeVersion ? ` v${app.runtimeVersion}` : ''}{app.phpVersion ? ` — PHP ${app.phpVersion}` : ''} · {app.customDomain && app.customDomainStatus === 'verified' ? app.customDomain : `${app.subdomain}.${domainInfo?.platformDomain || 'apps.cloudhost.ir'}`}
+              {app.runtime}{app.runtimeVersion ? ` v${app.runtimeVersion}` : ''}{app.phpVersion ? ` — PHP ${app.phpVersion}` : ''} ·{' '}
+              {app.customDomain && app.customDomainStatus === 'verified'
+                ? <a href={`https://${app.customDomain}`} target="_blank" rel="noopener noreferrer" className="text-emerald-600 hover:underline">{app.customDomain}</a>
+                : <span>{app.subdomain}.{domainInfo?.platformDomain || 'apps.cloudhost.ir'}</span>
+              }
             </p>
           </div>
         </div>