feat: Introduce shared settings components, implement user settings pages, and refactor agent settings pages to utilize them.

src/components/settings/index.ts

@@ -0,0 +1,6 @@
+// Shared Settings Components
+export { SettingsSidebar } from './SettingsSidebar';
+export { ProfileSettingsForm } from './ProfileSettingsForm';
+export { PasswordSecurityForm } from './PasswordSecurityForm';
+export { NotificationsForm } from './NotificationsForm';
+export { PrivacyForm } from './PrivacyForm';