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_encrypt.cpp

@@ -0,0 +1,20 @@
+#include <iostream>
+#include "src/core/models/account.h"
+
+int main() {
+    QString original = "mySecretPassword123!";
+    QString encrypted = Account::encryptPassword(original);
+    QString decrypted = Account::decryptPassword(encrypted);
+
+    std::cout << "Original: " << original.toStdString() << std::endl;
+    std::cout << "Encrypted: " << encrypted.toStdString() << std::endl;
+    std::cout << "Decrypted: " << decrypted.toStdString() << std::endl;
+
+    if (decrypted == original) {
+        std::cout << "SUCCESS: Encryption/decryption round-trip matches." << std::endl;
+        return 0;
+    } else {
+        std::cout << "FAIL: mismatch." << std::endl;
+        return 1;
+    }
+}