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));
+}

shared/protocol.ts

@@ -194,8 +194,10 @@ export const customDifficultySchema = z
     name: z
       .string()
       .trim()
-      .min(2)
-      .max(40)
+      .refine(
+        (name) => [...name].length >= 2 && [...name].length <= 40,
+        "Difficulty names must contain 2–40 characters",
+      )
       .refine(
         (name) =>
           !/[\u0000-\u001f\u007f-\u009f\u202a-\u202e\u2066-\u2069]/u.test(