chore(deps): update better-auth monorepo to v1.6.15

Author: renovate[bot]

package.json

@@ -73,7 +73,7 @@
     "@types/react-plotly.js": "2.6.4",
     "@types/semver": "7.7.1",
     "axios": "1.16.1",
-    "better-auth": "1.6.11",
+    "better-auth": "1.6.15",
     "dayjs": "1.11.20",
     "filesize": "11.0.17",
     "immer": "11.1.8",