fix(desktop): restore VITE_API_BASE for the desktop build

apps/web/.env.production's VITE_API_BASE went empty in 96fd97e to fix the
booth/browser same-origin case, but the desktop build shares that file and
was never given its own override — login broke with WebKitGTK's "The
string did not match the expected pattern." (a relative fetch() URL with
no base, from tauri://localhost). beforeBuildCommand now sets
VITE_API_BASE=http://127.0.0.1:3000 inline for the desktop build only;
verified both builds independently produce the right output.

apps/desktop/src-tauri/tauri.conf.json

@@ -7,7 +7,7 @@
     "devUrl": "http://localhost:5173",
     "frontendDist": "../../web/dist",
     "beforeDevCommand": "pnpm --filter @parking/web dev",
-    "beforeBuildCommand": "pnpm --filter @parking/web build"
+    "beforeBuildCommand": "VITE_API_BASE=http://127.0.0.1:3000 pnpm --filter @parking/web build"
   },
   "app": {
     "windows": [