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-character-section.tsx

@@ -35,7 +35,7 @@ export const PlayerCharacterSection = function PlayerCharacterSectionInner({
 					setEditingPlayer(undefined);
 					setManagementOpen(true);
 				}}
-				onSave={(name, ac, maxHp, color, icon) => {
+				onSave={(name, ac, maxHp, color, icon, level) => {
 					if (editingPlayer) {
 						editCharacter(editingPlayer.id, {
 							name,
@@ -43,9 +43,10 @@ export const PlayerCharacterSection = function PlayerCharacterSectionInner({
 							maxHp,
 							color: color ?? null,
 							icon: icon ?? null,
+							level: level ?? null,
 						});
 					} else {
-						createCharacter(name, ac, maxHp, color, icon);
+						createCharacter(name, ac, maxHp, color, icon, level);
 					}
 				}}
 				playerCharacter={editingPlayer}