fix: add empty postcss.config.js to override upstream clone's @tailwindcss/postcss config

The authcoreadmin Dockerfile clones AuthCoreAdmin upstream (which has
postcss.config.js referencing @tailwindcss/postcss) then COPY . . overlays
local files. Since the local build uses @tailwindcss/vite approach (no
postcss), the cloned postcss.config.js would fail to load
@tailwindcss/postcss (not installed). Adding an empty postcss.config.js
overrides the clone's version and silences the error.

Also adds the dialog class to Apps.vue modal overlay for E2E test
selector compatibility.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: liweinan

authcoreadmin/postcss.config.js

@@ -0,0 +1 @@
+export default {}