feat: create virtual devices (+ Add Device dropdown)

Backend:
- POST /api/create-null-sink {name} - loads null-sink module
- POST /api/create-loopback {name} - loads loopback module
- POST /api/unload-module {module_id} - unloads a module
- Fixed double-proxy-destroy crash in GraphEngine
- Graceful failure when module not available (no crash)

Frontend:
- + Add Device button in toolbar with dropdown menu
- Null Sink option (creates virtual audio output)
- Loopback Device option (creates paired input+output)
- Dropdown closes on outside click

Note: null-sink requires libpipewire-module-null-sink to be installed.
Loopback works on all PipeWire installations.

src/graph_engine.h

@@ -48,6 +48,10 @@ public:
     bool setNodeVolume(uint32_t node_id, float volume);
     bool setNodeMute(uint32_t node_id, bool mute);
 
+    // Module loading (virtual devices)
+    uint32_t loadModule(const char *name, const char *args);
+    bool unloadModule(uint32_t module_id);
+
     // PipeWire internal data (exposed for C callbacks)
     struct PwData {
         pw_thread_loop *loop;