[gnacho]

Summary

• Add an "Add Folder" button above the folder list in the main account view.
• Open the existing dialog for the active account and keep the existing validation and refresh flow.
• Remove the Preferences > Account row and its now-empty Synchronization Folders group, leaving the main view as the only Add Folder entry point.
• Remove the obsolete Settings callback and catalog string.

Tests

• Verify that the main-view button invokes the Add Folder callback.
• Verify that two accounts route the dialog to the active account and that no account presents no dialog.
• Verify that Preferences contains neither the Add Folder row nor its former group.
• cargo build
• cargo test: 767 passed, 0 failed, 3 ignored
• cargo clippy --all-targets -- -D warnings
• cargo fmt --all --check
• git diff --check
• i18n regeneration comparison: 538 entries, identical output

Closes #316