revert: remove PostCSS workaround plugin

CI passes on windows-latest with `ut build`, so the path doubling
is not a general turbopack Windows bug.  The PostCSS workaround was
targeting an unconfirmed symptom — remove it and keep only the
verified @source fix.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

postcss.config.js

@@ -1,35 +1,5 @@
-const path = require('path');
-const fs = require('fs');
-
-/**
- * Workaround for turbopack Windows path doubling bug.
- * On Windows, turbopack may pass a doubled relative path as the `from` option
- * to PostCSS (e.g. `./Desktop/demo/project/src/file.css` instead of `./src/file.css`),
- * causing `path.resolve(cwd, from)` to produce a non-existent doubled path.
- * This plugin detects and corrects the path before @tailwindcss/postcss reads it.
- * @see https://github.com/ant-design/ant-design-pro/issues/11747
- */
-const fixWindowsPathPlugin = {
-  postcssPlugin: 'fix-turbopack-windows-path',
-  Once(_root, { result }) {
-    const from = result.opts.from;
-    if (!from || fs.existsSync(from)) return;
-
-    const cwd = process.cwd();
-    const resolved = path.resolve(from);
-    const relative = path.relative(cwd, resolved);
-    const parts = relative.split(path.sep);
-
-    for (let i = 1; i < parts.length; i++) {
-      const candidate = path.join(cwd, ...parts.slice(i));
-      if (fs.existsSync(candidate)) {
-        result.opts.from = candidate;
-        return;
-      }
-    }
-  },
-};
-
 module.exports = {
-  plugins: [fixWindowsPathPlugin, require('@tailwindcss/postcss')],
+  plugins: {
+    '@tailwindcss/postcss': {},
+  },
 };