feat: Complete Preferences system implementation

- PreferencesService: QSettings-based persistence for all app settings
- StartupBehaviorService: Windows registry auto-start management with policy detection
- TranslationService: Language management with QTranslator, 9 built-in languages
- AiActionOptionsService: 30+ AI translation language options
- SettingsView: New 'Preferences' tab with full UI:
  * Search Mode (Local/Online)
  * Default App Mode (Mail/Calendar/Contacts/Settings)
  * Language selector (9 built-in + .qm files)
  * Email Sync Interval (1-1440 minutes)
  * Startup Behavior (auto-start, minimize to tray, notifications)
  * AI Translation (default language, summarize language, summary save path)
- MainMainWindow: Service wiring (PreferencesService, StartupBehaviorService, TranslationService, AiActionOptionsService)
- CMake: Added all new service files

All services integrated and wired to SettingsView.

src/ui/settingsview.h

@@ -17,6 +17,10 @@
 #include <QDialog>
 #include <QColorDialog>
 #include "services/accountservice.h"
+#include "services/preferencesservice.h"
+#include "services/startupbehaviorservice.h"
+#include "services/translationservice.h"
+#include "services/aiactionoptionsservice.h"
 #include "db/dao/common_structs.h"
 
 class SettingsView : public QWidget {
@@ -27,6 +31,10 @@ public:
     ~SettingsView() override = default;
 
     void setAccountService(AccountService *service);
+    void setPreferencesService(PreferencesService *service);
+    void setStartupBehaviorService(StartupBehaviorService *service);
+    void setTranslationService(TranslationService *service);
+    void setAiActionOptionsService(AiActionOptionsService *service);
 
 signals:
     void accountAddRequested();
@@ -92,6 +100,7 @@ private:
     QWidget* createAccountsTab();
     QWidget* createGeneralTab();
     QWidget* createAppearanceTab();
+    QWidget* createPreferencesTab();  // New: Preferences tab
     QWidget* createCategoriesTab();
     QWidget* createRulesTab();
     QWidget* createTemplatesTab();
@@ -149,4 +158,26 @@ private:
     QPushButton *m_signatureAddBtn;
     QPushButton *m_signatureEditBtn;
     QPushButton *m_signatureDeleteBtn;
+
+    // Services
+    AccountService *m_accountService = nullptr;
+    PreferencesService *m_preferencesService = nullptr;
+    StartupBehaviorService *m_startupBehaviorService = nullptr;
+    TranslationService *m_translationService = nullptr;
+    AiActionOptionsService *m_aiActionOptionsService = nullptr;
+
+    // Preferences tab controls
+    QComboBox *m_searchModeCombo = nullptr;
+    QComboBox *m_defaultAppModeCombo = nullptr;
+    QComboBox *m_languageCombo = nullptr;
+    QSpinBox *m_syncIntervalSpin = nullptr;
+    QCheckBox *m_startOnLoginCheck = nullptr;
+    QCheckBox *m_minimizeToTrayCheck = nullptr;
+    QCheckBox *m_enableNotificationsCheck = nullptr;
+    QComboBox *m_translateLanguageCombo = nullptr;
+    QComboBox *m_summarizeLanguageCombo = nullptr;
+    QLineEdit *m_summarySavePathEdit = nullptr;
+    QPushButton *m_summarySavePathBtn = nullptr;
+
+    int m_selectedAccountId = -1;
 };