Add custom notification sounds

[dorlugasigal]

Closes #758

Summary

Adds custom notification sounds to Settings.

You can import an AIFF, WAV, or CAF file under 30 seconds. Supacode validates it, keeps a managed copy, and uses it for both in-app and system notifications.

Type of change

• Bug fix (the linked issue is a bug report)
• Feature (the linked issue is a feature request marked ready)
• Documentation
• Other (please describe)

How was this tested?

• make check passes (format + lint)
• make test passes
• The app builds successfully

AI tool disclosure (optional)

• Model(s): GPT-5.6 Sol
• Harness / tools: GitHub Copilot CLI

Checklist

• This pull request is linked to an issue with Closes # above.
• For a feature, the linked issue is labeled ready.
• I am the author of this work and accountable for it; no commit is authored or co-authored by an AI agent.
• I have read the Contributing guide and the Code of Conduct.