Add folders from the main account view #316

The main synchronization view has no direct way to add a folder. The existing action is under Preferences > Account, behind the account selector added in #315. That flow works, but adding a folder to the account currently shown in the main window requires leaving the folder list and selecting the account again.

Add an "Add Folder" button to the main account view. It should call the existing active-account dialog and refresh path, so all validation and first-sync review behavior stays unchanged.

This is a workflow shortcut rather than a bug fix. Preferences already provides the functionality, so the change should stay small: reuse show_add_folder_dialog(), do not add another dialog implementation, and keep the existing Preferences > Account row.

Acceptance criteria:

• The main account view shows an "Add Folder" button near the folder list.
• Clicking it opens the existing dialog for active_account_id.
• With multiple accounts, the dialog uses the account currently shown in the main view.
• Without an active account, the action is unavailable.
• Completing the dialog uses the existing refresh_after_config_change() path.
• Preferences > Account remains unchanged.
• GTK tests cover the button and active-account selection.