Proofline 0.3.1

Fixed

• The installed and portable 0.3.0 apps could save a Claude API key but never read it back:
  the AI page kept showing the empty key box, Test stayed disabled, and AI features could not
  be used. A module that reading from Windows Credential Manager needs was missing from the
  package. A key you already saved in 0.3.0 is picked up after updating.
• When Proofline cannot read the key from Credential Manager, the AI page now says so under the
  key box. Before, the error went only to the log.