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/__tests__/factories/index.ts

@@ -1,2 +1,3 @@
 export { buildCombatant } from "./build-combatant.js";
+export { buildCreature } from "./build-creature.js";
 export { buildEncounter } from "./build-encounter.js";