Fix device selection: CPAL cannot open PipeWire sinks by name

ROOT CAUSE:
CPAL/Rodio (the audio library) cannot open PipeWire sinks by name directly.
CPAL only sees ALSA device names like "default", "hw:0,0", etc., not
PipeWire sink names like "alsa_output.usb-Cambridge_Audio...".

When we tried to pass the PipeWire sink name to reinit_audio_device(), CPAL
couldn't find it and fell back to system default, ignoring user selection.

SOLUTION:
1. When user selects a device, set it as PipeWire default using pactl
2. Pass device: null to reinit_audio_device() (uses PipeWire default)
3. On app startup, restore saved device as PipeWire default

This works because:
- PipeWire's "default" sink can be changed programmatically
- CPAL can successfully open "default" device
- The default sink points to the user's selected device

TRADE-OFFS:
- QBZ changes the system-wide PipeWire default sink
- Other apps will also use this sink (unless they specify their own)
- This is unavoidable with CPAL - it's the only way to select a specific
  PipeWire sink from Rust without using PipeWire/ALSA APIs directly

AFFECTED FUNCTIONS:
- handleOutputDeviceChange() - Sets sink as default when user selects device
- handleExclusiveModeChange() - Restores saved sink as default before reinit
- handleDacPassthroughChange() - Restores saved sink as default before reinit
- handleGaplessPlaybackChange() - Restores saved sink as default before reinit
- handleCrossfadeChange() - Restores saved sink as default before reinit
- loadAudioSettings() - Restores saved sink as default on app startup

src/lib/components/views/SettingsView.svelte

@@ -798,6 +798,15 @@
         // Look up the friendly description from the sink name
         const description = sinkNameToDescription.get(settings.output_device);
         outputDevice = description ?? settings.output_device;
+
+        // CRITICAL: Establish the saved device as PipeWire default on app start
+        // This ensures CPAL uses the correct device when playback begins
+        try {
+          await invoke('set_pipewire_default_sink', { sinkName: settings.output_device });
+          console.log('[Audio] Restored saved device as PipeWire default:', settings.output_device);
+        } catch (err) {
+          console.error('[Audio] Failed to set saved device as default:', err);
+        }
       } else {
         outputDevice = 'System Default';
       }
@@ -831,10 +840,15 @@
       // Save the preference
       await invoke('set_audio_output_device', { device: deviceToStore });
 
-      // Reinitialize audio to use the selected device
-      // IMPORTANT: Pass the actual sink name, not null
-      // This opens the specific device for QBZ without changing system default
-      await invoke('reinit_audio_device', { device: deviceToStore });
+      // CRITICAL: CPAL/Rodio cannot open PipeWire sinks by name directly
+      // We must set the sink as PipeWire default, then QBZ uses "default"
+      // This is a limitation of CPAL - it only sees ALSA devices, not PipeWire sinks
+      if (sinkName) {
+        await invoke('set_pipewire_default_sink', { sinkName });
+      }
+
+      // Reinitialize audio with default device (which now points to selected sink)
+      await invoke('reinit_audio_device', { device: null });
 
       console.log('Audio output device changed:', description, '(sink:', sinkName ?? 'default', ')');
     } catch (err) {
@@ -846,9 +860,15 @@
     exclusiveMode = enabled;
     try {
       await invoke('set_audio_exclusive_mode', { enabled });
-      // Reinitialize audio to apply/release exclusive mode
-      // IMPORTANT: Maintain the currently selected output device
-      await invoke('reinit_audio_device', { device: getCurrentDeviceSinkName() });
+
+      // Restore the currently selected device as PipeWire default
+      const sinkName = getCurrentDeviceSinkName();
+      if (sinkName) {
+        await invoke('set_pipewire_default_sink', { sinkName });
+      }
+
+      // Reinitialize audio (uses PipeWire default)
+      await invoke('reinit_audio_device', { device: null });
       console.log('Exclusive mode changed and audio reinitialized:', enabled);
     } catch (err) {
       console.error('Failed to change exclusive mode:', err);
@@ -867,9 +887,15 @@
 
     try {
       await invoke('set_audio_dac_passthrough', { enabled });
+
+      // Restore the currently selected device as PipeWire default
+      const sinkName = getCurrentDeviceSinkName();
+      if (sinkName) {
+        await invoke('set_pipewire_default_sink', { sinkName });
+      }
+
       // DAC passthrough may also require reinit for proper effect
-      // IMPORTANT: Maintain the currently selected output device
-      await invoke('reinit_audio_device', { device: getCurrentDeviceSinkName() });
+      await invoke('reinit_audio_device', { device: null });
       console.log('DAC passthrough changed and audio reinitialized:', enabled);
     } catch (err) {
       console.error('Failed to change DAC passthrough:', err);
@@ -885,8 +911,14 @@
       console.log('Gapless playback enabled: disabled DAC passthrough');
       try {
         await invoke('set_audio_dac_passthrough', { enabled: false });
-        // IMPORTANT: Maintain the currently selected output device
-        await invoke('reinit_audio_device', { device: getCurrentDeviceSinkName() });
+
+        // Restore the currently selected device as PipeWire default
+        const sinkName = getCurrentDeviceSinkName();
+        if (sinkName) {
+          await invoke('set_pipewire_default_sink', { sinkName });
+        }
+
+        await invoke('reinit_audio_device', { device: null });
       } catch (err) {
         console.error('Failed to disable DAC passthrough:', err);
       }
@@ -902,8 +934,14 @@
       console.log('Crossfade enabled: disabled DAC passthrough');
       try {
         await invoke('set_audio_dac_passthrough', { enabled: false });
-        // IMPORTANT: Maintain the currently selected output device
-        await invoke('reinit_audio_device', { device: getCurrentDeviceSinkName() });
+
+        // Restore the currently selected device as PipeWire default
+        const sinkName = getCurrentDeviceSinkName();
+        if (sinkName) {
+          await invoke('set_pipewire_default_sink', { sinkName });
+        }
+
+        await invoke('reinit_audio_device', { device: null });
       } catch (err) {
         console.error('Failed to disable DAC passthrough:', err);
       }