Fix bestiary creatures with zero HP silently failing to add

Bestiary sources like AWM store 0 for unknown HP. Passing maxHp: 0
into addCombatant triggered domain validation rejection, silently
dropping the creature. Treat hp: 0 as undefined, matching existing
ac: 0 handling.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

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

@@ -173,7 +173,7 @@ function addOneFromBestiary(
 
 	const id = combatantId(`c-${nextId + 1}`);
 	const result = addCombatantUseCase(store, id, newName, {
-		maxHp: entry.hp,
+		maxHp: entry.hp > 0 ? entry.hp : undefined,
 		ac: entry.ac > 0 ? entry.ac : undefined,
 		creatureId: cId,
 	});