Downgrade fumadocs to 15.x and update lockfile

Author: Rel1cx

package.json

@@ -111,6 +111,12 @@
       "@types/react": "^19.2.2",
       "@types/react-dom": "^19.2.2",
       "esbuild": "^0.25.11",
+      "fumadocs-core": "15.8.5",
+      "fumadocs-docgen": "3.0.2",
+      "fumadocs-mdx": "12.0.3",
+      "fumadocs-twoslash": "3.1.8",
+      "fumadocs-typescript": "4.0.11",
+      "fumadocs-ui": "15.8.5",
       "next": "^16.0.1",
       "react": "^19.2.0",
       "react-dom": "^19.2.0",