Rename and update vitest type declarations

Rename vitest.d.ts to vitest-setup.d.ts to avoid TypeScript module resolution
conflicts. Add custom matcher type declarations directly in this file following
Vitest documentation recommendations with import statement.

client/types/vitest-setup.d.ts

@@ -1,4 +1,13 @@
+import "vitest";
+
+// Custom matchers from tests/vitest/helpers.js
+interface CustomMatchers<R = unknown> {
+    toBeLocalized(): R;
+    toBeLocalizationOf(str: string): R;
+}
+
 declare module "vitest" {
+    // Export vitest functions for explicit imports (non-globals mode)
     export const describe: (typeof import("vitest/dist/index.js"))["describe"];
     export const it: (typeof import("vitest/dist/index.js"))["it"];
     export const test: (typeof import("vitest/dist/index.js"))["test"];
@@ -8,4 +17,8 @@ declare module "vitest" {
     export const afterEach: (typeof import("vitest/dist/index.js"))["afterEach"];
     export const beforeAll: (typeof import("vitest/dist/index.js"))["beforeAll"];
     export const afterAll: (typeof import("vitest/dist/index.js"))["afterAll"];
+
+    // Custom matcher type augmentations for Vitest 4.x
+    interface Assertion<T = any> extends CustomMatchers<T> {}
+    interface AsymmetricMatchersContaining extends CustomMatchers {}
 }