feat(whispering): add global volume control for sound effects

[nesquikm]

Summary

• Adds a sound.volume workspace KV setting (0–100%, default 100%) that controls the volume of all sound effects
• Sets HTMLAudioElement.volume before each sound plays in the query layer
• Adds a range slider to the sound settings page with live percentage display and preview sound on release

Closes #877

Test plan

• Open Settings > Sound, verify the volume slider appears at the top
• Drag the slider and release — a preview sound should play at the selected volume
• Set volume to ~30%, trigger a recording — confirm sound effects play at reduced volume
• Set volume to 0%, trigger a recording — confirm sounds are silent
• Set volume to 100%, confirm sounds play at full volume
• Verify per-sound toggles still work independently of the volume slider