Implement Credential transfer API (CXF-based credentials import / export)

### Checks

- [x] I have read the Wiki, searched the open issues, and still think this is a new feature.

### Explain the problem clearly and succinctly:

Migration from a password manager to another is tedious and may imply exporting / importing passwords in unencrypted files.

### Describe the solution you'd like:

A new Android API, [Credential transfer API](https://developer.android.com/identity/sign-in/credential-transfer), has been created to facilitate import / export of credentials from one password manager to another using the [FIDO Credential Exchange Format (CXF)](https://fidoalliance.org/specs/cx/cxf-v1.0-ps-20250814.html) standard.

It may help people using other password manager software to migrate to KeePassDX.

### Describe alternatives you've considered:

_No response_

### Additional context:

_No response_

Start by reading Android's Credential transfer API documentation and the linked FIDO Credential Exchange Format specification. No repository files, entry points, or tests are named, so first identify where KeePassDX handles credential import/export and Android integrations. Done should mean supporting CXF-based credential import and export through the new Android API.

Written by the indexing model from the issue text.