chore(deps): update @rsbuild/core and postcss (#55)

Update @rsbuild/core to `1.4.0-beta.1` and postcss to `8.5.4` to fix a
type issue in Rsbuild ecosystem CI:


https://github.com/rspack-contrib/rsbuild-ecosystem-ci/actions/runs/15461350377/job/43523200562

Author: chenjiahan

package.json

@@ -31,13 +31,13 @@
     "@biomejs/biome": "^1.9.4",
     "@playwright/test": "^1.52.0",
     "@rollup/pluginutils": "^5.1.4",
-    "@rsbuild/core": "^1.3.14",
+    "@rsbuild/core": "^1.4.0-beta.1",
     "@rslib/core": "^0.6.8",
     "@rsbuild/webpack": "^1.3.0",
     "@types/node": "^22.15.3",
     "@types/semver": "^7.7.0",
     "playwright": "^1.52.0",
-    "postcss": "^8.5.3",
+    "postcss": "^8.5.4",
     "semver": "^7.7.1",
     "simple-git-hooks": "^2.13.0",
     "tailwindcss": "^3.4.17",