keybind: macOS Cmd+J/K jump by prompt, Cmd+Backspace deletes word, expose all keybindings in config

- Cmd+K/J (and Cmd+[/]) now move up/down by prompt on macOS instead of
  line-scrolling; removed the legacy Command line-scroll fallback.
- Cmd/Super+Backspace/Delete now deletes the previous word (was delete-to-start),
  matching Alt/Option behavior, in both local and remote input paths.
- scroll_up_fallback/scroll_down_fallback now default to unset.
- Expose previously-hardcoded toggles in [keybindings]: side_panel_toggle,
  copy_selection_toggle, diagram_pane_toggle, typing_scroll_lock_toggle,
  diff_mode_cycle, info_widget_toggle (with env overrides and macOS Option
  character fallback preserved).
- Update help overlay descriptions and tests.