✅(frontend) switch to vitest and enhance testability

Migrated from jest to vitest for server/y-provider, gaining faster runs,
esm-native support and cleaner mocking.

Signed-off-by: Stephan Meijer <me@stephanmeijer.com>

src/frontend/servers/y-provider/vitest.config.mts

@@ -0,0 +1,11 @@
+import { URL, fileURLToPath } from 'url';
+
+import { defineConfig } from 'vitest/config';
+
+export default defineConfig({
+  test: {
+    alias: {
+      '@': fileURLToPath(new URL('./src', import.meta.url)),
+    },
+  },
+});