feat(07.1-01): expose user timezone on Better Auth session

- Adds `timezone` to additionalFields on the auth `user` config
- Default value reads process.env.DEFAULT_TIMEZONE (falls back to "UTC")
- session.user.timezone now available on every authenticated request
- Inferred User type automatically picks up the new field — no type changes needed

lib/auth.ts

@@ -92,6 +92,10 @@ export const auth = betterAuth({
         type: "boolean",
         defaultValue: false,
       },
+      timezone: {
+        type: "string",
+        defaultValue: process.env.DEFAULT_TIMEZONE || "UTC",
+      },
     },
   },
 });