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/cr-picker.tsx

@@ -0,0 +1,36 @@
+import { VALID_CR_VALUES } from "@initiative/domain";
+
+const CR_LABELS: Record<string, string> = {
+	"0": "CR 0",
+	"1/8": "CR 1/8",
+	"1/4": "CR 1/4",
+	"1/2": "CR 1/2",
+};
+
+function formatCr(cr: string): string {
+	return CR_LABELS[cr] ?? `CR ${cr}`;
+}
+
+export function CrPicker({
+	value,
+	onChange,
+}: {
+	value: string | null;
+	onChange: (cr: string | undefined) => void;
+}) {
+	return (
+		<select
+			className="rounded border border-border bg-card px-1.5 py-0.5 text-xs"
+			value={value ?? ""}
+			onChange={(e) => onChange(e.target.value || undefined)}
+			aria-label="Challenge rating"
+		>
+			<option value="">Assign</option>
+			{VALID_CR_VALUES.map((cr) => (
+				<option key={cr} value={cr}>
+					{formatCr(cr)}
+				</option>
+			))}
+		</select>
+	);
+}