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/components/turn-navigation.tsx

@@ -1,5 +1,7 @@
 import { Redo2, StepBack, StepForward, Trash2, Undo2 } from "lucide-react";
 import { useEncounterContext } from "../contexts/encounter-context.js";
+import { useDifficulty } from "../hooks/use-difficulty.js";
+import { DifficultyIndicator } from "./difficulty-indicator.js";
 import { Button } from "./ui/button.js";
 import { ConfirmButton } from "./ui/confirm-button.js";
 
@@ -15,6 +17,7 @@ export function TurnNavigation() {
 		canRedo,
 	} = useEncounterContext();
 
+	const difficulty = useDifficulty();
 	const hasCombatants = encounter.combatants.length > 0;
 	const isAtStart = encounter.roundNumber === 1 && encounter.activeIndex === 0;
 	const activeCombatant = encounter.combatants[encounter.activeIndex];
@@ -66,6 +69,7 @@ export function TurnNavigation() {
 				) : (
 					<span className="text-muted-foreground">No combatants</span>
 				)}
+				{difficulty && <DifficultyIndicator result={difficulty} />}
 			</div>
 
 			<div className="flex flex-shrink-0 items-center gap-3">