feat: add list clients menu option (#1382)

## Summary

- Add new "List existing users" option to management menu (option 2)
- Displays all client certificates with status (Valid/Revoked),
expiration date, and days remaining
- Reads expiry directly from certificate files using openssl for
accurate 4-digit year dates
- Output sorted by expiration date (oldest first)
- Updates test MENU_OPTION values to match new menu numbering

Example output:
```
=== Existing Clients ===

Found 2 certificate(s)

   Name                      Status     Expiry       Remaining
   ----                      ------     ------       ---------
   user1                     Valid      2035-12-11   3649 days
   user2                     Revoked    unknown      unknown
```

Closes #567
Closes #563
Closes #587

README.md

@@ -59,7 +59,8 @@ The first time you run it, you'll have to follow the assistant and answer a few
 When OpenVPN is installed, you can run the script again, and you will get the choice to:
 
 - Add a client
-- Remove a client
+- List client certificates
+- Revoke a client
 - Renew certificates (client or server)
 - Uninstall OpenVPN