fix: add trim() to username validation to prevent whitespace-only values

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

server/utils/auth/validation.ts

@@ -4,6 +4,7 @@ export const registerSchema = z
   .object({
     username: z
       .string()
+      .trim()
       .min(3, '用户名至少需要3个字符')
       .max(30, '用户名最多30个字符'),
     password: z.string().min(8, '密码至少需要8个字符'),