fix: improve GitHub Pages deployment reliability

- Add packageManager field to package.json for pnpm version consistency
- Remove explicit pnpm version from workflow (reads from packageManager)
- Add .nojekyll file to prevent Jekyll processing of _next directory
- Add .nojekyll generation step to deploy workflow

package.json

@@ -28,6 +28,7 @@
     "tailwind-merge": "^3.4.0",
     "zod": "^4.1.13"
   },
+  "packageManager": "[email protected]",
   "devDependencies": {
     "@tailwindcss/postcss": "^4.1.18",
     "@types/node": "^24.10.2",