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/__tests__/player-character-storage.test.ts

@@ -186,6 +186,38 @@ describe("player-character-storage", () => {
 			expect(loadPlayerCharacters()).toEqual([]);
 		});
 
+		it("preserves level through save/load round-trip", () => {
+			const pc = makePC({ level: 5 });
+			savePlayerCharacters([pc]);
+			const loaded = loadPlayerCharacters();
+			expect(loaded[0].level).toBe(5);
+		});
+
+		it("preserves undefined level through save/load round-trip", () => {
+			const pc = makePC();
+			savePlayerCharacters([pc]);
+			const loaded = loadPlayerCharacters();
+			expect(loaded[0].level).toBeUndefined();
+		});
+
+		it("discards character with invalid level", () => {
+			mockStorage.setItem(
+				STORAGE_KEY,
+				JSON.stringify([
+					{
+						id: "pc-1",
+						name: "Test",
+						ac: 10,
+						maxHp: 50,
+						color: "blue",
+						icon: "sword",
+						level: 25,
+					},
+				]),
+			);
+			expect(loadPlayerCharacters()).toEqual([]);
+		});
+
 		it("keeps valid characters and discards invalid ones", () => {
 			mockStorage.setItem(
 				STORAGE_KEY,