feat: remember last used contact source when importing

[niklas-e]

Type of change(s)

• Bug fix
• Feature / enhancement
• Infrastructure / tooling (CI, build, deps, tests)
• Documentation

What changed and why

The import dialog already defaulted to the last used contact source but never stored the selection, so it kept falling back to whatever was set elsewhere.

Note: lastUsedContactSource is shared with the new-contact screen, so an import now also updates the default account there. This is consistent with the pref's existing "last used" meaning, but it does touch the behaviour discussed in #9.

Tests performed

• Last used source is persisted in same session
• Last used source is persisted when restarting the app

Closes the following issue(s)

• Closes Import: remeber last used Contacts DB #81

Checklist

• I read the contribution guidelines.
• I manually tested my changes on device/emulator (if applicable).
• I updated the "Unreleased" section in CHANGELOG.md (if applicable).
• I have self-reviewed my pull request (no typos, formatting errors, etc.).
• I understand every change in this pull request.