fix: restore frontend source code (src/web/) that was accidentally deleted

Frontend source is needed for development/building the SPA.
Only Node.js runtime server code should be removed.

src/web/pages/sites.mobile.test.tsx

@@ -0,0 +1,10 @@
+import { describe, expect, it } from 'vitest';
+import { readFileSync } from 'node:fs';
+import { resolve } from 'node:path';
+
+describe('Sites mobile layout', () => {
+  it('includes mobile-card usage in Sites page', () => {
+    const source = readFileSync(resolve(process.cwd(), 'src/web/pages/Sites.tsx'), 'utf8');
+    expect(source).toContain('mobile-card');
+  });
+});