Validate and deduplicate custom difficulty presets

shared/difficulty-presets.ts

@@ -0,0 +1,13 @@
+import type { DifficultyPreset } from "./protocol";
+
+// A list fetched before a save must not remove the newly saved preset.
+export function mergeDifficultyPresets(
+  current: DifficultyPreset[],
+  incoming: DifficultyPreset[],
+) {
+  return [
+    ...new Map(
+      [...current, ...incoming].map((preset) => [preset.id, preset]),
+    ).values(),
+  ].sort((a, b) => a.name.localeCompare(b.name));
+}