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/air-freight/page/server-reference-manifest.json

@@ -0,0 +1,17 @@
+{
+  "node": {
+    "40275f0c2193420edd4a0270f91d8fd12134ede38f": {
+      "workers": {
+        "app/air-freight/page": {
+          "moduleId": 67113,
+          "async": false,
+          "exportedName": "sendEmailAction",
+          "filename": "src/actions/sendEmail.ts"
+        }
+      },
+      "exportedName": "sendEmailAction",
+      "filename": "src/actions/sendEmail.ts"
+    }
+  },
+  "edge": {}
+}