Tasks: close workspaces from the GUI

• T001 Add a failing display test for sidebar targeting, cancel, confirm, and refresh in crates/signaltty-gui/src/app_tests.rs.
• T002 Add the row control in sidebar.rs, the menu action in actions.rs, and the confirmation and IPC call in app.rs.
• T003 Update GUI documentation and verify light and dark appearance.
• T004 Run fmt, clippy, and all workspace tests.