fix(calendar): SSRF exception for Exchange + error messages + domain in edit

- SSRF check skipped for Exchange type (internal EWS servers are common)
- testConnectionFromConfig catches SSRF/validation errors, returns {success:false,error} instead of throwing 403
- updateSource reads existing.type to determine effective type for SSRF check
- Panel shows saveError/editSaveError on failed add/update
- Edit form initialValues now includes domain field
- i18n: calendar.saveError key added (de+en)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

apps/web/src/messages/de.json

@@ -208,6 +208,7 @@
       "formTesting": "Teste...",
       "formTestSuccess": "Verbindung erfolgreich",
       "formTestFailed": "Verbindung fehlgeschlagen",
+      "saveError": "Speichern fehlgeschlagen. Bitte Eingaben prüfen.",
       "formSave": "Speichern",
       "formSaving": "Wird gespeichert...",
       "formCancel": "Abbrechen",