cli: Switch CSS bundling from lightningcss to postcss-import

postcss-import naturally preserves @layer declarations during bundling,
eliminating the need for custom layer parsing and restoration logic.
This simplifies the implementation while also preserving source comments.

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>
Co-authored-by: Cursor <cursoragent@cursor.com>

packages/cli/package.json

@@ -110,7 +110,6 @@
     "inquirer": "^8.2.0",
     "jest-css-modules": "^2.1.0",
     "json-schema": "^0.4.0",
-    "lightningcss": "^1.29.1",
     "lodash": "^4.17.21",
     "minimatch": "^9.0.0",
     "node-stdlib-browser": "^1.3.1",
@@ -119,6 +118,7 @@
     "p-queue": "^6.6.2",
     "pirates": "^4.0.6",
     "postcss": "^8.1.0",
+    "postcss-import": "^16.1.0",
     "process": "^0.11.10",
     "raw-loader": "^4.0.2",
     "react-dev-utils": "^12.0.0-next.60",