fix: use safer email validation regex to prevent ReDoS

Replace the vulnerable email regex with a safer pattern that:
- Uses explicit character class limits instead of negated classes
- Limits local part to 1-64 characters
- Limits domain to 1-255 characters
- Limits TLD to 2-6 characters
- Prevents catastrophic backtracking attacks

Author: XiaoMing0000

src/tools/x509-certificate-generator/x509-certificate-generator.vue

@@ -320,7 +320,7 @@ const emailValidation = useValidation({
   source: email,
   rules: [
     {
-      validator: value => !value || /^[^\s@]+@[^\s@]+\.[^\s@]+$/.test(value),
+      validator: value => !value || /^[^@\s]{1,64}@[^@\s]{1,255}\.[^@\s]{2,6}$/.test(value),
       message: t('tools.x509-certificate-generator.invalidEmailFormat'),
     },
   ],