fix: add pagefind for static export search functionality

- Add pagefind as devDependency (^1.2.1)
- Add postbuild script to generate search index
- Add _pagefind/ to .gitignore
- Configure pagefind to index .next/server/app and output to out/_pagefind

This fixes the production search error where pagefind.js was not found.
With static export, pagefind needs to run post-build to generate the
search index from the built HTML files.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: bernatfp

.gitignore

@@ -9,6 +9,7 @@
 # Next.js
 /.next/
 /out/
+_pagefind/
 
 # Production
 /build

package.json

@@ -6,6 +6,7 @@
   "scripts": {
     "dev": "next dev",
     "build": "next build",
+    "postbuild": "pagefind --site .next/server/app --output-path out/_pagefind",
     "start": "next start",
     "export": "next build",
     "lint": "next lint",
@@ -36,6 +37,7 @@
     "eslint-plugin-jsx-a11y": "^6.10.2",
     "husky": "^9.1.7",
     "lint-staged": "^15.2.11",
+    "pagefind": "^1.2.1",
     "prettier": "^3.4.2",
     "typescript": "^5.7.2"
   },