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/persistence/encounter-storage.ts

@@ -72,6 +72,9 @@ export function loadEncounter(): Encounter | null {
 					? validConditions
 					: undefined;
 
+			// Validate isConcentrating field
+			const isConcentrating = entry.isConcentrating === true ? true : undefined;
+
 			// Validate and attach HP fields if valid
 			const maxHp = entry.maxHp;
 			const currentHp = entry.currentHp;
@@ -85,12 +88,13 @@ export function loadEncounter(): Encounter | null {
 					...base,
 					ac: validAc,
 					conditions,
+					isConcentrating,
 					maxHp,
 					currentHp: validCurrentHp ? currentHp : maxHp,
 				};
 			}
 
-			return { ...base, ac: validAc, conditions };
+			return { ...base, ac: validAc, conditions, isConcentrating };
 		});
 
 		const result = createEncounter(