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/components/difficulty-indicator.tsx

@@ -13,16 +13,29 @@ const TIER_CONFIG: Record<
 
 const BAR_HEIGHTS = ["h-2", "h-3", "h-4"] as const;
 
-export function DifficultyIndicator({ result }: { result: DifficultyResult }) {
+export function DifficultyIndicator({
+	result,
+	onClick,
+}: {
+	result: DifficultyResult;
+	onClick?: () => void;
+}) {
 	const config = TIER_CONFIG[result.tier];
 	const tooltip = `${config.label} encounter difficulty`;
 
+	const Element = onClick ? "button" : "div";
+
 	return (
-		<div
-			className="flex items-end gap-0.5"
+		<Element
+			className={cn(
+				"flex items-end gap-0.5",
+				onClick && "cursor-pointer rounded p-1 hover:bg-muted/50",
+			)}
 			title={tooltip}
 			role="img"
 			aria-label={tooltip}
+			onClick={onClick}
+			type={onClick ? "button" : undefined}
 		>
 			{BAR_HEIGHTS.map((height, i) => (
 				<div
@@ -34,6 +47,6 @@ export function DifficultyIndicator({ result }: { result: DifficultyResult }) {
 					)}
 				/>
 			))}
-		</div>
+		</Element>
 	);
 }