chore(frontend): consolidate duplicate vitest config files

frontend/src/__tests__/vitest-config.test.ts

@@ -0,0 +1,20 @@
+import { describe, expect, it } from 'vitest';
+import fs from 'fs';
+import path from 'path';
+
+// Regression guard for https://github.com/LabsCrypt/flowfi/issues/1281:
+// having both vitest.config.ts and vitest.config.mts at the same time makes
+// the resolved test config ambiguous (Vite/Vitest pick one based on
+// extension resolution order, which is not obvious to contributors and can
+// silently disable the "other" config's environment, setup file, and
+// coverage thresholds). Only one vitest.config.* should ever exist.
+describe('vitest config', () => {
+  it('has exactly one vitest.config.* file in the frontend package', () => {
+    const frontendRoot = path.resolve(__dirname, '..', '..');
+    const configFiles = fs
+      .readdirSync(frontendRoot)
+      .filter((f) => /^vitest\.config\.(ts|mts|cts|js|mjs|cjs)$/.test(f));
+
+    expect(configFiles).toEqual(['vitest.config.ts']);
+  });
+});