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/hooks/use-encounter.ts

@@ -9,6 +9,7 @@ import {
 	setAcUseCase,
 	setHpUseCase,
 	setInitiativeUseCase,
+	toggleConcentrationUseCase,
 	toggleConditionUseCase,
 } from "@initiative/application";
 import type {
@@ -204,6 +205,19 @@ export function useEncounter() {
 		[makeStore],
 	);
 
+	const toggleConcentration = useCallback(
+		(id: CombatantId) => {
+			const result = toggleConcentrationUseCase(makeStore(), id);
+
+			if (isDomainError(result)) {
+				return;
+			}
+
+			setEvents((prev) => [...prev, ...result]);
+		},
+		[makeStore],
+	);
+
 	return {
 		encounter,
 		events,
@@ -217,5 +231,6 @@ export function useEncounter() {
 		adjustHp,
 		setAc,
 		toggleCondition,
+		toggleConcentration,
 	} as const;
 }