fix: stop .gitignore from excluding build/.next (site 502/blank after deploy)

.gitignore's ".next/" rule (no leading slash) matched at any depth, so
build/.next was silently excluded from every commit even though build.sh
populates it. The Docker image was built without the compiled server/static
output, so server.js had nothing to serve. Anchor the rule to the repo root
and commit the compiled output that Dokploy actually needs.

build/.next/server/app/contact/page/react-loadable-manifest.json

@@ -0,0 +1,9 @@
+{
+  "42391": {
+    "id": 42391,
+    "files": [
+      "static/chunks/0t48hzs_6fshe.css",
+      "static/chunks/0ht_jt035hxfz.js"
+    ]
+  }
+}