feat: Online search button

- MailListView: new '🌐 Online' button in search bar
- Clicking button emits onlineSearchRequested(query)
- If search box empty, shows QInputDialog for query entry
- MainMainWindow::onOnlineSearchRequested() handler:
  - Validates current folder selected
  - Gets account for folder
  - Shows status message
  - Shows info dialog with query/folder/account (placeholder for IMAP SEARCH)
- Signal: onlineSearchRequested(const QString& query)
- Button styled in blue (online theme)

src/ui/mainmainwindow.cpp

@@ -1118,3 +1118,34 @@ void MainMainWindow::onEmbeddedDetachRequested(QWidget *widget)
 
     statusBar()->showMessage(tr("Compose view detached to separate window"), 3000);
 }
+
+void MainMainWindow::onOnlineSearchRequested(const QString& query)
+{
+    if (m_currentFolderId < 0) {
+        QMessageBox::warning(this, "Buscar online", "Selecciona una carpeta primero");
+        return;
+    }
+    
+    std::optional<Folder> optFolder = FolderDao::findById(m_currentFolderId);
+    if (!optFolder.has_value()) return;
+    
+    Folder folder = optFolder.value();
+    Account* account = m_accountService->findAccountById(folder.accountId());
+    if (!account) {
+        QMessageBox::warning(this, "Buscar online", "No se encontró la cuenta para esta carpeta");
+        return;
+    }
+    
+    // Use MailService to search online (IMAP SEARCH)
+    statusBar()->showMessage(tr("Buscando en servidor: %1").arg(query), 3000);
+    
+    // TODO: Implement IMAP SEARCH in MailService
+    // For now, show a message
+    QMessageBox::information(this, "Buscar online", 
+        QString("Búsqueda en servidor para: '%1'\nCarpeta: %2\nCuenta: %3")
+            .arg(query)
+            .arg(folder.name())
+            .arg(account->email()));
+    
+    delete account;
+}