Show image captions in desktop notifications

[tayfuryldz]

Closes #733

What changed

• show an image's caption in desktop notifications when Matrix provides a distinct filename
• keep the existing generic notification when body is only the filename
• add focused tests for caption and filename-only cases

Why

Matrix uses body as a user-written media caption when filename is present and differs from it. iamb currently discards that caption and always shows sent an image.

Testing

• focused helper cases are included as unit tests
• full cargo test was not run locally because this change was prepared without downloading the repository/dependencies; CI should provide the repository build/test validation

[tayfuryldz]

Updated both points: captions are now labeled by attachment type (for example, Image: Sunset), and the same caption handling covers audio, file, image, and video messages. Filename-only attachments keep the existing generic notification text.

[VAWVAW]

Please rebase this branch so I can merge it.