[audio] persistent device volume control (#4416)

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- [x] I have read and followed the [AI Policy](https://git.eden-emu.dev/eden-emu/eden/src/branch/master/docs/policies/AI.md)
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-------------------
So, upon investigating AC3 not booting when fw 20+ and overlay applet disabled, i found the leads:
"LibAppletOff" and "AudioTvOutput". By further decoding instructions i've found a guest assert comparing device volume with hdmi volume. Since we implemented no  proper master volume when no streams are created the assert were failing causing guest to panic (why?????????)
Anyway. ac3 now works fw20+ and no overlay applet.. Maybe other games too.

Reviewed-on: https://git.eden-emu.dev/eden-emu/eden/pulls/4416
Reviewed-by: lizzie <lizzie@eden-emu.dev>
Reviewed-by: MaranBr <maranbr@eden-emu.dev>

src/audio_core/sink/null_sink.h

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: Copyright 2026 Eden Emulator Project
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 // SPDX-FileCopyrightText: Copyright 2018 yuzu Emulator Project
 // SPDX-License-Identifier: GPL-2.0-or-later
 
@@ -38,6 +41,7 @@ public:
                                   StreamType type) override {
         if (null_sink == nullptr) {
             null_sink = std::make_unique<NullSinkStreamImpl>(system, type);
+            null_sink->SetDeviceVolume(device_volume);
         }
         return null_sink.get();
     }
@@ -45,9 +49,14 @@ public:
     void CloseStream(SinkStream*) override {}
     void CloseStreams() override {}
     f32 GetDeviceVolume() const override {
-        return 1.0f;
+        return device_volume;
+    }
+    void SetDeviceVolume(f32 volume) override {
+        device_volume = volume;
+        if (null_sink != nullptr) {
+            null_sink->SetDeviceVolume(volume);
+        }
     }
-    void SetDeviceVolume(f32 volume) override {}
     void SetSystemVolume(f32 volume) override {}
 
 private: