Add encounter difficulty indicator (5.5e XP budget)

Live 3-bar difficulty indicator in the top bar showing encounter
difficulty (Trivial/Low/Moderate/High) based on the 2024 5.5e XP
budget system. Automatically derived from PC levels and bestiary
creature CRs.

- Add optional level field (1-20) to PlayerCharacter
- Add CR-to-XP and XP Budget per Character lookup tables in domain
- Add calculateEncounterDifficulty pure function
- Add DifficultyIndicator component with color-coded bars and tooltip
- Add useDifficulty hook composing encounter, PC, and bestiary contexts
- Indicator hidden when no PCs with levels or no bestiary-linked monsters
- Level field in PC create/edit forms, persisted in storage

Closes #18

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

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

@@ -0,0 +1,54 @@
+import type {
+	Combatant,
+	CreatureId,
+	DifficultyResult,
+	PlayerCharacter,
+} from "@initiative/domain";
+import { calculateEncounterDifficulty } from "@initiative/domain";
+import { useMemo } from "react";
+import { useBestiaryContext } from "../contexts/bestiary-context.js";
+import { useEncounterContext } from "../contexts/encounter-context.js";
+import { usePlayerCharactersContext } from "../contexts/player-characters-context.js";
+
+function derivePartyLevels(
+	combatants: readonly Combatant[],
+	characters: readonly PlayerCharacter[],
+): number[] {
+	const levels: number[] = [];
+	for (const c of combatants) {
+		if (!c.playerCharacterId) continue;
+		const pc = characters.find((p) => p.id === c.playerCharacterId);
+		if (pc?.level !== undefined) levels.push(pc.level);
+	}
+	return levels;
+}
+
+function deriveMonsterCrs(
+	combatants: readonly Combatant[],
+	getCreature: (id: CreatureId) => { cr: string } | undefined,
+): string[] {
+	const crs: string[] = [];
+	for (const c of combatants) {
+		if (!c.creatureId) continue;
+		const creature = getCreature(c.creatureId);
+		if (creature) crs.push(creature.cr);
+	}
+	return crs;
+}
+
+export function useDifficulty(): DifficultyResult | null {
+	const { encounter } = useEncounterContext();
+	const { characters } = usePlayerCharactersContext();
+	const { getCreature } = useBestiaryContext();
+
+	return useMemo(() => {
+		const partyLevels = derivePartyLevels(encounter.combatants, characters);
+		const monsterCrs = deriveMonsterCrs(encounter.combatants, getCreature);
+
+		if (partyLevels.length === 0 || monsterCrs.length === 0) {
+			return null;
+		}
+
+		return calculateEncounterDifficulty(partyLevels, monsterCrs);
+	}, [encounter.combatants, characters, getCreature]);
+}