fix: restore SDL volume to MAX after device switch

Remove dead 'currentVolume' variable that was never written to, causing
SwitchDevice to always restore volume to 0 after switching audio devices.

- Remove currentVolume from package state
- SwitchDevice now sets mix.MAX_VOLUME after reopening device
- GetVolume128 now queries SDL directly instead of reading stale variable
- Add unit tests for parseSinks, parseSinkLine, and ConvertVolume
- Add changelog.txt tracking all audio bug fixes

changelog.txt

@@ -0,0 +1,37 @@
+# Changelog
+
+All notable changes to this project will be documented in this file.
+
+The format is based on [Keep a Changelog](https://keepachangelog.com/).
+
+## [Unreleased]
+
+### Fixed
+- Audio volume silently zeroed after switching output devices
+- Device switch not persisted to database across restarts
+- Device restore path passes invalid device name to SDL
+- Stop() no-op when audio toggled off mid-playback
+- Active device highlight incorrect when using default device
+- Play() deadlocks when called before device is open
+
+### Added
+- Human-readable audio device names (Built-in Speakers, HDMI/TV/Monitor)
+- Unit tests for mixer package (parseSinks, parseSinkLine, ConvertVolume)
+
+### Changed (Major Refactors)
+- Mixer package: inject audio backend dependencies for testability
+- Mixer package: extract wpctl calls behind an interface
+
+## [Audio Bugs v0.1] - 2026-07-10
+
+### Fixed
+- Audio volume silently zeroed after switching output devices
+- Device switch not persisted to database across restarts
+- Device restore path passes invalid device name to SDL
+- Stop() no-op when audio toggled off mid-playback
+- Active device highlight incorrect when using default device
+- Play() deadlocks when called before device is open
+
+### Added
+- Human-readable audio device names (Built-in Speakers, HDMI/TV/Monitor)
+- Unit tests for mixer package (parseSinks, parseSinkLine, ConvertVolume)