fix: pin better-auth to ~1.4.x to fix build:site

The ^1.4.15 specifier allowed better-auth 1.5.6 to be installed,
which removed the createAuthMiddleware export and broke the docs build.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: nperez0111

docs/package.json

@@ -72,7 +72,7 @@
     "@vercel/analytics": "^1.6.1",
     "@y-sweet/react": "^0.6.3",
     "ai": "^6.0.5",
-    "better-auth": "^1.4.15",
+    "better-auth": "~1.4.15",
     "better-sqlite3": "^12.6.2",
     "class-variance-authority": "^0.7.1",
     "framer-motion": "^12.26.2",