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/AudioOptions.go

@@ -273,9 +273,22 @@ func (p *AudioOptions) SelectAudioDevice(deviceIndex int) error {
 		return fmt.Errorf("invalid device index: %d", deviceIndex)
 	}
 	device := p.AudioDevices[deviceIndex]
-	if err := mixer.SwitchDevice(device.Label); err != nil {
+
+	config, err := db.GetConfig(p.Context.DbPtr)
+	if err != nil {
+		return fmt.Errorf("failed to get config for device switch: %w", err)
+	}
+
+	if err := mixer.SwitchDevice(device.Label, config); err != nil {
 		return fmt.Errorf("failed to switch to device %s: %w", device.Label, err)
 	}
+
+	// Persist the selected device to the database
+	config.SelectedAudioDevice = device.Label
+	if err := p.Context.DbPtr.Save(config).Error; err != nil {
+		logger.Error("failed to persist device selection: %v", err)
+	}
+
 	p.activeDeviceIndex = deviceIndex
 	return nil
 }