feat(admin-email): SMTP test-send card on /admin/email

Adds a plaintext-only SMTP connectivity test on the email-settings
page. Distinct from the branding-preview "Send a test" affordance:

  - branding-preview exercises the full rendering pipeline (logo +
    branded shell + colour) — useful for confirming the email *looks*
    right.
  - this test isolates SMTP — minimal HTML, plaintext alternative, no
    logo dependency — so a failure is purely transport. Confirms the
    configured credentials (env or per-port DB) reach the wire before
    a real notification flow depends on them.

SMTP errors surface inline below the input (auth failure, ENOTFOUND,
connection refused, etc.) rather than as a passing toast — the whole
point of the test is to read them.

`/api/v1/admin/email/test-send` route reuses `sendEmail(...,
ctx.portId)` so per-port SMTP overrides are exercised the same way a
real notification would.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/app/(dashboard)/[portSlug]/admin/email/page.tsx

@@ -2,6 +2,7 @@ import { PageHeader } from '@/components/shared/page-header';
 import { RegistryDrivenForm } from '@/components/admin/shared/registry-driven-form';
 import { SalesEmailConfigCard } from '@/components/admin/sales-email-config-card';
 import { EmailRoutingCard } from '@/components/admin/email-routing-card';
+import { SmtpTestSendCard } from '@/components/admin/email/smtp-test-send-card';
 
 export default function EmailSettingsPage() {
   return (
@@ -23,6 +24,7 @@ export default function EmailSettingsPage() {
         title="SMTP transport overrides"
         description="Optional per-port SMTP credentials. Leave blank to use the global env defaults. Each field shows its current source (env / port / default) so you can tell what's active without checking the deploy."
       />
+      <SmtpTestSendCard />
       <SalesEmailConfigCard />
       <EmailRoutingCard />
     </div>