Add XDG support for metadata directory

[lb803]

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This PR

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• Code is tested (provide details).

References

• Issue Enhance Standard Directory Support for Configuration #221

Additional information

This addresses an issue where offlineimap did not automatically evaluate XDG conventions for the metadata directory location.

While users could manually specify XDG paths in the config file, the code never proactively checked them. Following the existing pattern from init.py (lines 216-229), we now check for XDG_DATA_HOME before falling back to ~/.offlineimap.

The implementation follows a light touch approach: XDG locations are evaluated first, but users can still override via configuration. No XDG enforcement is added. Patch includes unit tests: baseline, regression and new feature coverage.