fix(desktop): pin empty PostCSS config so Vite stops walking up the home tree (#40609)

Salvaged from #40526; re-verified on main, tightened, tested.

Co-authored-by: xxxigm <xxxigm@users.noreply.github.com>

apps/desktop/vite.config.ts

@@ -6,6 +6,19 @@ import path from 'path'
 export default defineConfig({
   base: './',
   plugins: [react(), tailwindcss()],
+  css: {
+    // Pin an explicit (empty) PostCSS config. Tailwind is handled entirely by
+    // `@tailwindcss/vite`, so the renderer needs no PostCSS plugins — and
+    // without this, Vite's `postcss-load-config` walks UP the filesystem
+    // looking for a stray `postcss.config.*` / `tailwind.config.*`. The desktop
+    // build runs from inside the user's home tree (e.g.
+    // `C:\Users\<name>\AppData\Local\hermes\hermes-agent\apps\desktop`), so an
+    // unrelated Tailwind v3 config higher up the tree gets picked up and
+    // reprocesses our v4 stylesheet, failing the build with
+    // "`@layer base` is used but no matching `@tailwind base` directive is
+    // present." Pinning the config makes the build hermetic.
+    postcss: { plugins: [] }
+  },
   build: {
     // Keep desktop packaging stable: Shiki ships many dynamic chunks by
     // default, and electron-builder can OOM scanning thousands of files.