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/build-creature.ts

@@ -0,0 +1,26 @@
+import type { Creature } from "@initiative/domain";
+import { creatureId } from "@initiative/domain";
+
+let counter = 0;
+
+export function buildCreature(overrides?: Partial<Creature>): Creature {
+	const id = ++counter;
+	return {
+		id: creatureId(`creature-${id}`),
+		name: `Creature ${id}`,
+		source: "srd",
+		sourceDisplayName: "SRD",
+		size: "Medium",
+		type: "humanoid",
+		alignment: "neutral",
+		ac: 13,
+		hp: { average: 7, formula: "2d6" },
+		speed: "30 ft.",
+		abilities: { str: 10, dex: 14, con: 10, int: 10, wis: 10, cha: 10 },
+		cr: "1/4",
+		initiativeProficiency: 0,
+		proficiencyBonus: 2,
+		passive: 10,
+		...overrides,
+	};
+}