feat(07-06): settings-api client + SmtpSettingsForm + SMTP page (DKV-05)

- settings-api.ts: fetchSmtp/saveSmtp/testSmtp with credentials:'include'; SmtpConfig exposes only hasPassword (T-07-17)
- smtp-settings-form.tsx: Surface C form with show/hide password toggle, test-feedback auto-clears 6s
- settings/general/smtp/page.tsx: SmtpSettingsPage heading + SmtpSettingsForm
- de.json + en.json: add smtp.showPassword/hidePassword/testTesting/testSuccess/testFailed keys
- TDD GREEN: 5/5 tests pass; aria-hidden* on required markers removed for correct getByLabelText matching

apps/web/src/messages/en.json

@@ -130,7 +130,12 @@
       "fromAddress": "Sender Address (From)",
       "fromAddressHelp": "The email address shown as the sender.",
       "save": "Save Settings",
-      "test": "Test Connection"
+      "test": "Test Connection",
+      "showPassword": "Show password",
+      "hidePassword": "Hide password",
+      "testTesting": "Testing connection...",
+      "testSuccess": "Connection successful",
+      "testFailed": "Connection failed"
     }
   },
   "widgets": {