removing keys at failed encryption and decryption

[LucaBonamino]

Description

• Cleaned up the public key text field and file input in the finally block of the encryption handler. Applied the same cleanup to the private key inputs in the decryption handler.
• dumped the manifest version to 0.1.2

Type of change

Please check the relevant option:

• Bug fix
• New feature
• Performance improvement
• Documentation update
• Refactor / cleanup
• Security-related change

Changes made

• Cleaned up the public key text field and file input in the finally block of the encryption handler. Applied the same cleanup to the private key inputs in the decryption handler.
• dumped the manifest version to 0.1.2

Testing

Describe how you tested these changes.

• Existing tests pass
• New tests were added or updated, if applicable
• Manual testing was performed
• Gmail-specific behavior was tested, if applicable

Checklist

• The change has been tested manually in Gmail, if applicable.
• Relevant automated tests have been added or updated.
• Documentation has been updated if behavior changed.
• No private keys, email content, or other sensitive data are included.
• No new permissions, external services, or dependencies were added without explanation.
• Any security or privacy implications have been documented.

Related issues

Closes #3