feat(settings): auto-switch to newly connected outputs

[XiaKaiYang]

What changed

• added a new Auto-Switch New Output audio setting so newly connected output devices can become the active output immediately when the user wants that behavior
• updated output-device connection handling and pending auto-switch reconciliation so FineTune accepts the new device when the setting is enabled instead of forcing the old default back
• documented the new setting in the troubleshooting guide and added focused tests for the switching rules plus AppSettings persistence

Why

FineTune currently only auto-switches to a connected output when that device is already the highest-priority device. Newly seen outputs are appended to the end of the priority list, so common cases like plugging in headphones require a manual menu bar selection every time.

Impact

• users can opt into immediate switching for newly connected outputs without changing the existing priority-based behavior for everyone else
• existing disconnect fallback and priority ordering behavior remain unchanged when the new setting is off

Validation

• xcodebuild test -project /Users/xiakaiyang/WeChatReplyAssistant/tmp/FineTune/FineTune.xcodeproj -scheme FineTune -destination 'platform=macOS' CODE_SIGNING_ALLOWED=NO CODE_SIGNING_REQUIRED=NO CODE_SIGN_IDENTITY='' -only-testing:FineTuneTests/OutputDeviceAutoSwitchTests
• xcodebuild test -project /Users/xiakaiyang/WeChatReplyAssistant/tmp/FineTune/FineTune.xcodeproj -scheme FineTune -destination 'platform=macOS' CODE_SIGNING_ALLOWED=NO CODE_SIGNING_REQUIRED=NO CODE_SIGN_IDENTITY='' -only-testing:FineTuneTests/SettingsJSONTests -only-testing:FineTuneTests/AppSettingsDefaultTests

[gigimon]

Super feature, I need it :)