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/build-manifest.json

@@ -0,0 +1,22 @@
+{
+  "pages": {
+    "/_app": []
+  },
+  "devFiles": [],
+  "polyfillFiles": [
+    "static/chunks/03~yq9q893hmn.js"
+  ],
+  "lowPriorityFiles": [
+    "static/VkXP3ZSqJcnzcJArhJHd2/_buildManifest.js",
+    "static/VkXP3ZSqJcnzcJArhJHd2/_ssgManifest.js",
+    "static/VkXP3ZSqJcnzcJArhJHd2/_clientMiddlewareManifest.js"
+  ],
+  "rootMainFiles": [
+    "static/chunks/098a0lm-b~rn..js",
+    "static/chunks/0tldj~reb5~dn.js",
+    "static/chunks/110q6k5sdl4es.js",
+    "static/chunks/14_678uuyq.tf.js",
+    "static/chunks/07uz2g0_38qia.js",
+    "static/chunks/turbopack-11jn_eqn~72wc.js"
+  ]
+}