feat(audio): Add per-output L/R stereo balance

[macedot]

Summary

• Adds per-output L/R stereo balance (0.0 = full left, 0.5 = center, 1.0 = full right), persisted by device UID
• Hardware devices use CoreAudio VirtualMainBalance; software/DDC-backed devices apply macOS-style L/R gains in the process-tap DSP path
• Re-applies persisted balance after FineTune volume writes so System Settings balance is no longer reset to center

Fixes #16

Test plan

• On a hardware stereo output, drag the new L/R balance slider and confirm audible pan
• Change device volume in FineTune and confirm balance stays where you set it (does not snap to center)
• Quit and relaunch FineTune; confirm per-device balance is restored
• Double-click the balance slider to reset to center
• On a software-backed output (no native volume), confirm balance still works for apps FineTune routes
• Confirm unit tests for StereoBalance mapping / settings round-trip / buffer balance pass