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.

.forgejo/workflows/build-prod.yml

@@ -17,6 +17,11 @@ jobs:
       - name: Checkout code
         uses: actions/checkout@v6
 
+      - name: Test
+        run: |
+          . /nix/var/nix/profiles/default/etc/profile.d/nix-daemon.sh
+          nix develop --command go test ./internal/mixer/ -v -timeout 30s
+
       - name: Build
         run: |
           . /nix/var/nix/profiles/default/etc/profile.d/nix-daemon.sh