feat: responsive compact mail view + folder unread badges

- CompactMailDelegate: card-based view for narrow panels (<420px)
  * Avatar with hash-based color, sender + date (relative format)
  * Subject line with attachment icon
  * Action buttons: flag, delete, category, more (context menu)
  * Unread highlight with light blue background + bold sender
- MailListView: automatic switch between tree/table (wide) and list/cards (narrow)
  * QStackedWidget with QTreeView + QListView sharing same proxy model
  * resizeEvent threshold at 420px
  * Signals forwarded to MainMainWindow for actions
- FolderTreeDelegate: custom paint for folder tree
  * Unread count badge (blue pill) right-aligned on folder nodes
  * Account nodes bold, folder nodes normal weight
  * Proper indentation per depth level
- MailItemDao::countUnreadByFolderId() for real-time unread counts
- MainMainWindow handlers for compact actions (flag, delete, category, more menu)

test_delete_account.cpp

@@ -0,0 +1,62 @@
+#include <QCoreApplication>
+#include <QDebug>
+#include "src/services/accountservice.h"
+#include "src/db/dao/accountdao.h"
+#include "src/db/databasemanager.h"
+
+int main(int argc, char *argv[])
+{
+    QCoreApplication app(argc, argv);
+    // Ensure DB is initialized
+    DatabaseManager::instance().database(); // triggers init if needed
+
+    AccountService service;
+
+    // Create a test account
+    Account testAcc;
+    testAcc.setEmail("testdelete@example.com");
+    testAcc.setDisplayName("Test Delete");
+    testAcc.setType(AccountType::IMAP);
+    Account::ConnectionSettings settings;
+    settings.incomingHost = "imap.example.com";
+    settings.incomingPort = 993;
+    settings.incomingSsl = true;
+    settings.outgoingHost = "smtp.example.com";
+    settings.outgoingPort = 465;
+    settings.outgoingSsl = true;
+    settings.username = "testdelete@example.com";
+    settings.password = "testpass";
+    settings.authMethod = "LOGIN";
+    testAcc.setConnectionSettings(settings);
+
+    qDebug() << "Adding test account...";
+    service.addAccount(testAcc);
+    qint64 addedId = testAcc.id(); // after addAccount, id should be set via DAO insert returning id
+    qDebug() << "Added account ID:" << addedId;
+
+    // Verify exists
+    QVector<Account> accounts = AccountDao::findAll();
+    qDebug() << "Accounts after add:" << accounts.size();
+    for (const Account &acc : accounts) {
+        qDebug() << "  ID:" << acc.id() << "Email:" << acc.email();
+    }
+
+    // Remove account
+    qDebug() << "Removing account ID:" << addedId;
+    service.removeAccount(addedId);
+
+    // Verify removed
+    accounts = AccountDao::findAll();
+    qDebug() << "Accounts after remove:" << accounts.size();
+    for (const Account &acc : accounts) {
+        qDebug() << "  ID:" << acc.id() << "Email:" << acc.email();
+    }
+
+    if (accounts.isEmpty()) {
+        qDebug() << "SUCCESS: No accounts remain.";
+        return 0;
+    } else {
+        qDebug() << "FAILURE: Accounts still present.";
+        return 1;
+    }
+}