fix(deps): move next 16.3.6 -> 16.3.8, the patch release that fixes six OSV advisories

apps/docs pins next exactly; better-auth's optional next peer resolved
to the same version. Both resolutions now name 16.3.8. The lockfile
change is regenerated by pnpm and moves only next, @next/env and the
eight @next/swc-* binaries.

Advisories fixed in 16.3.8 (OSV ranges, fixed: 16.3.8):
GHSA-mcj8-r9mp-w47p, GHSA-f87g-xv8r-7p7x, GHSA-cjq9-62q9-8jv4,
GHSA-4jqv-mc3x-m676, GHSA-3w37-wq28-93x7, GHSA-39w2-rjm5-chcv.

Claude-Session: https://claude.ai/code/session_01VF48aw8RPG6wzDnMgp6rtw
Co-authored-by: Claude <noreply@anthropic.com>

Author: claude

apps/docs/package.json

@@ -18,7 +18,7 @@
     "fumadocs-ui": "16.14.4",
     "lucide-react": "^1.31.0",
     "mermaid": "^11.16.0",
-    "next": "16.3.6",
+    "next": "16.3.8",
     "next-themes": "^0.4.6",
     "react": "^19.2.8",
     "react-dom": "^19.2.8",