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/hooks/use-difficulty.ts

@@ -29,9 +29,12 @@ function deriveMonsterCrs(
 ): string[] {
 	const crs: string[] = [];
 	for (const c of combatants) {
-		if (!c.creatureId) continue;
-		const creature = getCreature(c.creatureId);
-		if (creature) crs.push(creature.cr);
+		if (c.creatureId) {
+			const creature = getCreature(c.creatureId);
+			if (creature) crs.push(creature.cr);
+		} else if (c.cr) {
+			crs.push(c.cr);
+		}
 	}
 	return crs;
 }