Volume control for added audio

[elroychibex]

Is your feature request related to a problem? Please describe.
After adding a background audio file, there was no way to reduce it's volume. Hence, the audio was louder than my voice in the recording.

Describe the solution you'd like
Ability to reduce the volume of the added audio.

Ideally, a volume slider that appears after an audio is added

[meiiie]

Implemented in #354 and merged into main.

Added-audio regions now have an Audio Volume panel when selected on the timeline. The slider uses the existing persisted/exported AudioRegion.volume path, clamps valid values to 0-100%, and ignores invalid input rather than resetting to full volume.

I verified preview/UI behavior with a production Electron smoke and export behavior with 100% vs 0% audio-level smoke.

Closing as implemented. If the next build still lacks the control in a specific flow, please reopen or file a focused follow-up.