Reassign jump_to_prompt from Ctrl+Shift+PageUp/PageDown to
Ctrl+Shift+Arrow Up/Down on GTK, freeing the idiomatic Linux
keybinds (Ctrl+Shift+PageUp/PageDown) for move_tab.
This matches the tab-moving keybinds used by Firefox, GNOME Terminal,
and VSCode. The new jump_to_prompt binding mirrors the macOS pattern
(Cmd+Shift+Arrow Up/Down).
Closes#4998
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>