Built-in macro steps for Enter, Backspace, Delete, Select All, Copy, Paste

These are all achievable today via "Press Key" but they're not discoverable. Add a new MACRO_STEP_TYPES entry "Keystroke" that exposes a curated dropdown of named keystrokes: Enter, Tab, Escape, Backspace, Delete, Space, Arrow keys, Home/End/PageUp/PageDown, Ctrl+A, Ctrl+C, Ctrl+V, Ctrl+X, Ctrl+Z, Ctrl+Y. Maps onto existing SendInput infrastructure with no new low-level code. Could also alias 'Submit' → Enter for clarity.