feat: Search Online (IMAP) + real Calendar/Contacts UI + iCloud auth wiring

Search Online:
- Synchronizer::searchOnline(folderId, query) virtual with default {}
- ImapSynchronizer: SELECT + UID SEARCH (Subject/From/To/TEXT) + fetch results
- MailService::searchOnline() + onlineSearchFinished/onlineSearchError signals
- MainMainWindow::onOnlineSearchRequested shows real server results

Calendar:
- Real QCalendarWidget view with per-day agenda from local mails
- New MailItemDao::findByDateRange(start, end)

Contacts:
- QTableWidget aggregating addresses from all local mails
- Regex email extraction + live search filter

iCloud/AccountService:
- Wire IcloudAuthenticator into AccountService::createAuthenticator

src/ui/signaturemanagerdialog.cpp

@@ -275,4 +275,3 @@ void SignatureManagerDialog::onSignatureSelected(QListWidgetItem* item)
         m_selectedSignatureHtml = item->data(Qt::UserRole + 1).toString();
     }
 }
-SignatureManagerDialog::~SignatureManagerDialog() = default;