Support token-only otp field?

**Is your feature request related to a problem? Please describe.**
If the `otp` field is a raw token (not otpauth://), KeePassDX does not show otp code.

**Describe the solution you'd like**
I tried `otpauth://totp/?secret={token}` without issuer and username, and it worked, so can we support `{token}` only?

**Additional context**
Typically I copy the raw token from the issuer website, and save into Keepass, and don't make it `otpauth://`. Both the Windows KeePass (with KeeTrayTOTP plugin) and KeePassXC support raw token, so I wish KeePassDX can support it.
Thanks!

Review the existing handling of otpauth:// values and trace where the otp field is parsed and displayed. Define the raw-token case from the issue, then verify that saving a token without the otpauth:// prefix results in a visible OTP code while existing otpauth:// behavior remains supported.

Written by the indexing model from the issue text.