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/persistence/player-character-storage.ts

@@ -44,6 +44,14 @@ export function rehydrateCharacter(raw: unknown): PlayerCharacter | null {
 		return null;
 	if (!isValidOptionalMember(entry.color, VALID_PLAYER_COLORS)) return null;
 	if (!isValidOptionalMember(entry.icon, VALID_PLAYER_ICONS)) return null;
+	if (
+		entry.level !== undefined &&
+		(typeof entry.level !== "number" ||
+			!Number.isInteger(entry.level) ||
+			entry.level < 1 ||
+			entry.level > 20)
+	)
+		return null;
 
 	return {
 		id: playerCharacterId(entry.id),
@@ -52,6 +60,7 @@ export function rehydrateCharacter(raw: unknown): PlayerCharacter | null {
 		maxHp: entry.maxHp,
 		color: entry.color as PlayerCharacter["color"],
 		icon: entry.icon as PlayerCharacter["icon"],
+		level: entry.level,
 	};
 }