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/App.tsx

@@ -76,6 +76,9 @@ function useActionBarAnimation(combatantCount: number) {
 export function App() {
 	const {
 		encounter,
+		isEmpty,
+		hasCreatureCombatants,
+		canRollAllInitiative,
 		advanceTurn,
 		retreatTurn,
 		addCombatant,
@@ -211,14 +214,6 @@ export function App() {
 		sidePanel.showCreature,
 	]);
 
-	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 (
 		<div className="flex h-screen flex-col">
 			<div className="relative mx-auto flex w-full max-w-2xl flex-1 flex-col gap-3 px-4 min-h-0">