Add Crit Damage button for dying/stable participants

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).

TODO.md

@@ -6,13 +6,6 @@ Backlog of bugs + long-term items. Milestones live in REWORK_PLAN.md.
 
 
 
-### FEAT: critical damage button for dying/stable participants
-- Shared logic already supports `applyHpChange(..., { isCriticalHit:true })`.
-- UI only has normal Damage, which adds 1 failure at 0 HP.
-- Add button near Revive/Stabilize: **Crit Damage** / **Critical Hit**.
-- Use only for dying/stable character/NPC at 0 HP; adds 2 death-save failures.
-- Dead target remains dead; no action.
-
 ### dm list - keep active particpant in view (scroll)
 
 ### combat.js doesnt seem to actually exercise the add characters.  there are no characters in the campaign.   only in the encounter..