Move derived encounter flags into useEncounter() hook

Relocate isEmpty, hasCreatureCombatants, and canRollAllInitiative
from App.tsx into useEncounter(), reducing inline derivations in
the component (Phase 5 of App decomposition plan).

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

apps/web/src/hooks/use-encounter.ts

@@ -371,9 +371,20 @@ export function useEncounter() {
 		[makeStore, editCombatant],
 	);
 
+	const isEmpty = encounter.combatants.length === 0;
+	const hasCreatureCombatants = encounter.combatants.some(
+		(c) => c.creatureId != null,
+	);
+	const canRollAllInitiative = encounter.combatants.some(
+		(c) => c.creatureId != null && c.initiative == null,
+	);
+
 	return {
 		encounter,
 		events,
+		isEmpty,
+		hasCreatureCombatants,
+		canRollAllInitiative,
 		advanceTurn,
 		retreatTurn,
 		addCombatant,