5e crit-at-0-HP rule: any crit damage while downed = +2 death-save failures.
Shared logic supported via applyHpChange(..., { isCriticalHit: true }) but UI
had no trigger. Normal Damage at 0 only added 1 fail.
handleCritDamage handler calls applyHpChange with isCriticalHit: true.
Button renders in HP block (right-aligned) for character/NPC participants
when dying or stable and combat started. Dead excluded (no-op).