chore(deps): pin postcss override to exact 8.5.19

Use an exact postcss version in overrides instead of ^8.5.19,
matching the repo convention and addressing review feedback on
#504.

Co-authored-by: samanyugoyal2010 <samanyugoyal2010@users.noreply.github.com>

Author: cursoragent

apps/livekit-moss-vercel/agent-react/package.json

@@ -55,7 +55,7 @@
   },
   "pnpm": {
     "overrides": {
-      "postcss": "^8.5.19",
+      "postcss": "8.5.19",
       "sharp": "0.35.3"
     }
   },

apps/moss-llamaindex/frontend/package.json

@@ -34,7 +34,7 @@
     "@tailwindcss/typography": "^0.5.19"
   },
   "overrides": {
-    "postcss": "^8.5.19",
+    "postcss": "8.5.19",
     "sharp": "0.35.3"
   }
 }

apps/next-js/package.json

@@ -37,7 +37,7 @@
     "vitest": "^4.1.8"
   },
   "overrides": {
-    "postcss": "^8.5.19",
+    "postcss": "8.5.19",
     "sharp": "0.35.3"
   }
 }