[anyapipira]

Summary

Add a BGM volume control to the MMD dance dialog so background audio does not start at full system volume by default.

Behavior

• Place the volume slider directly beside the audio start-offset control.
• Show volume as an integer percentage with a 0% to 100% range.
• Default new and existing installations without a saved value to 50%.
• Apply volume changes immediately to an active XAudio2 source voice.
• Reapply the configured gain whenever playback creates a new source voice, including after seeking, looping, output-device changes, or restarting playback.
• Persist the volume in UI preferences and validate/clamp loaded values.
• Lay out the offset and volume controls inline without the previous fixed-column overlap risk for longer localized labels.

Localization and documentation

• Add the Volume label in English, Simplified Chinese, and Japanese.
• Document the 50% default, supported range, and preference persistence in all three MMD guides.
• Clarify in all three building guides that Media Foundation, Core Audio/MMDevice, and XAudio2 2.9 are Windows/Windows SDK system dependencies. They require no separate audio SDK or redistributable on the supported Windows 10+ runtime and are not bundled third-party DLLs.

Verification

• Built AnyaDanceUI in Release configuration.
• Built anyadance_tests in Release configuration.
• Ran ctest --test-dir build -C Release --output-on-failure: all tests passed.
• Added audio-player coverage for the 50% default and volume clamping.