feat: configurable custom audio feedback sounds

[boeserwolf]

Before Submitting This PR

Please confirm you have done the following:

• I have searched existing issues and pull requests (including closed ones) to ensure this isn't a duplicate
• I have read CONTRIBUTING.md

If this is a feature or change that was previously closed/rejected:

• I have explained in the description below why this should be reconsidered
• I have gathered community feedback (link to discussion below)

Human Written Description

I wanted to keep Handy’s existing audio feedback behavior, but make it possible to use personal sounds for recording start and stop.
This adds optional custom start/stop sound file selection in settings without removing the built-in sound themes.
The goal is to make feedback sounds more flexible while keeping the default flow unchanged for existing users.

Related Issues/Discussions

Fixes #
Discussion:

Community Feedback

No separate discussion thread yet.
This change was tested directly in a real Linux desktop workflow and is backward compatible with existing sound themes.

Testing

• Verified built-in themes still work as before.
• Enabled audio feedback and selected custom file for Start.
• Enabled audio feedback and selected custom file for Stop.
• Confirmed custom sounds play on recording start/stop.
• Confirmed removing custom files falls back to built-in theme sounds.
• Confirmed preview controls work for both custom and built-in sounds.
• Built and tested desktop app locally (tauri build + AppImage run).

Screenshots/Videos (if applicable)

N/A

AI Assistance

• No AI was used in this PR
• AI was used (please describe below)

If AI was used:

• Tools used: Codex (gpt-5.3-codex)
• How extensively: Assisted with implementation, refactoring, and PR text formatting.

[cjpais]

I'll review this when I get a chance, but if you could, could you rebase?

[boeserwolf]

Thanks a lot! Rebase is ready.