feat(quick-260914-m97): Feld Fehlermeldungen an im SMTP-Formular — settings-api, Formular, i18n settings.smtp

- SmtpConfig.bugReportRecipient (string | null) und SaveSmtpPayload.bugReportRecipient (null loescht, fehlend bewahrt)
- Eingabefeld type=email hinter der Absenderadresse mit Hinweistext; Payload traegt immer trim() || null
- Zwei Komponententests: Vorbelegung aus GET, PUT-Payload mit Wert bzw. null
- i18n settings.smtp.bugReportRecipient / bugReportRecipientHelp in de und en

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018N9CD3ebPKm1b32bPpBknY

apps/web/src/messages/en.json

@@ -168,7 +168,9 @@
       "testToHelp": "Optional — sends a real test email to this address.",
       "testTesting": "Testing connection...",
       "testSuccess": "Connection successful",
-      "testFailed": "Connection failed"
+      "testFailed": "Connection failed",
+      "bugReportRecipient": "Bug reports to",
+      "bugReportRecipientHelp": "Optional – mailbox that receives the reports users send via the \"Report a problem\" button, including the screenshot. Leave empty if the button should not send e-mails."
     }
   },
   "widgets": {