Add manual CR assignment and difficulty breakdown panel

Implement issue #21: custom combatants can now have a challenge rating
assigned via a new breakdown panel, opened by tapping the difficulty
indicator. Bestiary-linked combatants show read-only CR with source name;
custom combatants get a CR picker with all standard 5e values. CR persists
across reloads and round-trips through JSON export/import.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

apps/web/src/persistence/__tests__/encounter-storage.test.ts

@@ -134,6 +134,26 @@ describe("loadEncounter", () => {
 		expect(loadEncounter()).toBeNull();
 	});
 
+	it("round-trip preserves combatant cr field", () => {
+		const result = createEncounter(
+			[
+				{
+					id: combatantId("c-1"),
+					name: "Custom Thug",
+					cr: "2",
+				},
+			],
+			0,
+			1,
+		);
+		if (isDomainError(result)) throw new Error("unreachable");
+		saveEncounter(result);
+		const loaded = loadEncounter();
+
+		expect(loaded).not.toBeNull();
+		expect(loaded?.combatants[0].cr).toBe("2");
+	});
+
 	it("saving after modifications persists the latest state", () => {
 		const encounter = makeEncounter();
 		saveEncounter(encounter);