Implement the 023-clear-encounter feature that adds a clear encounter button with confirmation dialog to remove all combatants and reset round/turn counters, with the cleared state persisting across page refreshes

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

packages/domain/src/events.ts

@@ -98,6 +98,11 @@ export interface ConcentrationEnded {
 	readonly combatantId: CombatantId;
 }
 
+export interface EncounterCleared {
+	readonly type: "EncounterCleared";
+	readonly combatantCount: number;
+}
+
 export type DomainEvent =
 	| TurnAdvanced
 	| RoundAdvanced
@@ -113,4 +118,5 @@ export type DomainEvent =
 	| ConditionAdded
 	| ConditionRemoved
 	| ConcentrationStarted
-	| ConcentrationEnded;
+	| ConcentrationEnded
+	| EncounterCleared;