feat: add threat analysis panel to Review mode

- New Engine Threat engine (selectable via Engine->Set Engine Threat)
- Toggle button in Review mode runs null-move analysis showing what
  the opponent would play if the side to move were to pass
- PV limited to 5 plies; suppressed when side to move is in check
- Move list / book panel height reduced from 11 to 8 to make room
- Threat panel placed above the existing engine analysis section
- Shares navigation debounce with regular analysis for smooth UX

Agent-Logs-Url: https://github.com/fsmosca/Python-Easy-Chess-GUI/sessions/bb73f816-a9a3-4e9e-ae6e-8273f36dc02b

Co-authored-by: fsmosca <22366935+fsmosca@users.noreply.github.com>

Author: Copilot