auth dialog: tell the human the keyboard works (Tab / arrows / Enter / Esc)

`_add_button_row` (`src/doberman/auth/gui_prompter.py:331-424`) wires Tab and the arrow keys to move the highlight between Deny and Approve and Enter to invoke the highlighted button (`target.bind("", _move_highlight)` at line 419), and Escape/closing the window denies. Nothing on screen says so; the buttons are custom canvas shapes, so there's no native focus ring to hint at it. **What to do** 1. Below the button row, draw one short muted line (`_MUTED`, `_SUB_FONT` or `_DEADLINE_FONT`): `Tab / arrows switch - Enter confirms - Esc denies`. Keep it ASCII; the auth prompt text has an ASCII-only guarantee with a test behind it, and a middle dot would trip it. 2. Include its height in the canvas/panel height calculation so the window grows by one line instead of clipping. 3. Test in `tests/unit/test_gui_prompter.py` with the faked canvas: after the dialog is populated, a text item containing `Enter` exists. Cosmetic. The bindings themselves don't change, and the deny-first highlight stays.