Stabilize sidequest pane sizing

[WBT112]

Summary

• make the tmux game pane height deterministic on startup and window resize
• keep the command pane visible while letting Snake grow up to its useful max height
• compact Game Over and highscore panels in short panes so score, name entry, and exit controls remain visible
• align the enforced minimum terminal height with the compact layout at 80x22

Validation

• go clean -testcache && ./scripts/qa.sh --race --cover --vuln