Implement the 018-combatant-concentration feature that adds a per-combatant concentration toggle with Brain icon, purple border accent, and damage pulse animation in the encounter tracker

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/web/src/App.tsx

@@ -16,6 +16,7 @@ export function App() {
 		adjustHp,
 		setAc,
 		toggleCondition,
+		toggleConcentration,
 	} = useEncounter();
 
 	return (
@@ -53,6 +54,7 @@ export function App() {
 							onAdjustHp={adjustHp}
 							onSetAc={setAc}
 							onToggleCondition={toggleCondition}
+							onToggleConcentration={toggleConcentration}
 						/>
 					))
 				)}