chore: migrate from pnpm to bun package manager

Switch package manager from pnpm to bun for faster dependency
installation and better monorepo support. Remove pnpm-specific
configuration files and update all scripts to use bun commands.

package.json

@@ -1,10 +1,10 @@
 {
   "name": "enchun-monorepo",
   "private": true,
-  "packageManager": "pnpm@10.17.0",
+  "packageManager": "bun@1.3.3",
   "scripts": {
     "dev": "turbo run dev --parallel",
-    "dev:stop": "echo 'Stopping dev servers...' && pkill -f 'astro.js dev' && pkill -f 'next dev' && pkill -f 'pnpm dev' && echo 'Dev servers stopped' || echo 'No dev servers were running'",
+    "dev:stop": "echo 'Stopping dev servers...' && pkill -f 'astro dev' && pkill -f 'next dev' && echo 'Dev servers stopped' || echo 'No dev servers were running'",
     "build": "turbo run build",
     "lint": "turbo run lint",
     "test": "turbo run test",
@@ -17,13 +17,6 @@
     "eslint-plugin-react-hooks": "^7.0.1",
     "turbo": "^2.0.5"
   },
-  "pnpm": {
-    "onlyBuiltDependencies": [
-      "sharp",
-      "esbuild",
-      "unrs-resolver"
-    ]
-  },
   "workspaces": [
     "apps/frontend",
     "apps/backend",