audio interface: separate input/output device selectors

- Added Output Device dropdown alongside Input Device
- loadAudioDevices() now populates both dropdowns from the API and
  selects current values
- Separated setAudioInputDevice / setAudioOutputDevice functions
  (each calls API with only its respective field)
- Added input_device / output_device to GET /api/audio/profile response
- Removed the old combined setAudioDevice function

src/web/server.py

@@ -1353,6 +1353,8 @@ class WebServer:
                 "rate": profile["rate"],
                 "jack_running": True,
                 "xrun_count": xruns,
+                "input_device": self.deps.audio_system.config.input_device,
+                "output_device": self.deps.audio_system.config.output_device,
             }
 
         @app.post("/api/audio/profile")