fix: resolve 4 critical audio mixer bugs (v0.5)

- SwitchDevice volume silently zeroed: Init uses cfg.SavedVolume,
  SwitchDevice restores saved volume after reopening device
- SwitchDevice not persisted to database: caller saves SelectedAudioDevice
- Device restore passes invalid 'system default' to SDL: now passes ''
- Stop() no-op when audio toggled off: removed AudioEnabled guard

Regression tests added for all 4 bugs. CI now runs go test before build.

internal/page/home.go

@@ -13,7 +13,7 @@ import (
 	"rhemawavetech.com/rhemawave/internal/shaderManager"
 )
 
-var GitVersion = "Audio-Bug-Fix 0.1"
+var GitVersion = "Audio-Bug-Fix 0.5"
 
 type Home struct {
 	Base