[Feature] Add option to duck music player volume instead of pausing (#541)

[LoackyBit]

Description

This PR implements the auto-ducking feature requested in issue #541. When enabled, instead of pausing the selected music player when other audio starts, Background Music will temporarily duck the music player's volume, restoring it when silence returns.

Key Changes

• Auto-duck Toggle: Added a checkable "Duck instead of pause" menu item under the Auto-pause [Selected Player] Preferences submenu. When active, the main menu title dynamically changes from "Auto-pause [Selected Player]" to "Auto-duck [Selected Player]".
• Ducking Volume Slider: Added a dynamic NSSlider for Ducking Volume in the Preferences submenu to allow users to set the ducking volume level (0% to 100%, defaults to 30%).
• Reactive UI: The Ducking Volume slider dynamically enables/disables depending on whether the auto-duck option is checked. Slider adjustments update currently-ducked player volumes in real time.
• Audio State Transitions & Fixes:
  • Since ducked music players continue playing audio, the device's audible state transitions to kBGMDeviceIsSilentExceptMusic instead of kBGMDeviceIsSilent when other audio stops. The listener block now correctly handles this state to trigger the unducking process.
  • Initialized and defaulted fallback volumes to 50 (the neutral midpoint raw volume in the driver curve). This fixes an issue where a 50% ducking level resulted in a no-op, and prevents unducking from boosting the player volume to 100 (400% volume boost).
  • Added a tolerance range ($\pm 1$) when comparing the current volume to the target ducked volume to account for float-to-int rounding differences by CoreAudio.

Type of Change

• New feature

Related Issues

Closes #541

How to Test

1. Launch Background Music and navigate to Preferences -> Auto-pause [Selected Player].
2. Select your music player of choice.
3. Check the Duck instead of pause option and verify that the main menu label updates to Auto-duck [Selected Player].
4. Play music on the selected player, then play audio from another app (e.g. YouTube in Chrome).
5. Verify that the player's volume ducks to the level specified by the new Ducking Volume slider.
6. Verify that changing the Ducking Volume slider while the music is ducked updates its volume in real time.
7. Stop the other app's audio and verify that the player's volume is restored to its exact pre-ducked volume.
8. Manually adjust the player's volume while ducked. Stop the other audio and verify that the manual adjustment is preserved as the new baseline volume.

[LoackyBit]

@kyleneideck can you review and merge the PR?

[kyleneideck]

Hi @LoackyBit , thanks for the PR. I'm going to try to go through the PR backlog in order. I've been testing #853 and made some changes on top of it, but I think it's pretty close. So I'm not sure how long until I get to this one, but hopefully fairly soon.

I had a quick look through and left a few minor comments, but overall it looks good so far.

[LoackyBit]

Thanks for the review and the feedback @kyleneideck. I’ve already pushed a new commit implementing your suggestions. No rush on the merge, just let me know if you spot anything else I should improve.