Replace imperative matches/when functions with declarative KeyCombo and
ShortcutWhen data structures, laying groundwork for a future rebinding UI.
Split all isMod (metaKey || ctrlKey) bindings into explicit platform pairs:
- Mac: Cmd (meta) only — Ctrl is never intercepted, passes through to terminal
- Linux/Windows: Ctrl — disabled when terminal is focused so Ctrl+W, Ctrl+K,
etc. reach vim/shell