Show media captions in desktop notifications

[tayfuryldz]

Closes #733

This is the rebased follow-up to #746 after #747 landed.

What changed

• show labeled captions for audio, file, image, and video attachments when Matrix provides a distinct filename
• keep the existing generic notification when body is only the filename
• add focused regression tests for caption and filename-only cases

Testing

Focused unit tests are included. Full cargo test was not run locally because the branch was prepared without downloading the repository/dependencies; CI provides repository build/test validation.