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/player-management.tsx

@@ -68,6 +68,11 @@ export function PlayerManagement({
 								<span className="text-muted-foreground text-xs tabular-nums">
 									HP {pc.maxHp}
 								</span>
+								{pc.level !== undefined && (
+									<span className="text-muted-foreground text-xs tabular-nums">
+										Lv {pc.level}
+									</span>
+								)}
 								<Button
 									variant="ghost"
 									size="icon-sm"