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.

frontend/src/lib/stores.ts

@@ -383,4 +383,47 @@ export async function setNodeMute(nodeId: number, mute: boolean) {
   }
 }
 
+// Virtual devices
+export async function createNullSink(name: string): Promise<number | null> {
+  try {
+    const res = await fetch('/api/create-null-sink', {
+      method: 'POST',
+      headers: { 'Content-Type': 'application/json' },
+      body: JSON.stringify({ name }),
+    });
+    const data = await res.json();
+    return data.module_id || null;
+  } catch (e) {
+    console.error('[api] create-null-sink failed:', e);
+    return null;
+  }
+}
+
+export async function createLoopback(name: string): Promise<number | null> {
+  try {
+    const res = await fetch('/api/create-loopback', {
+      method: 'POST',
+      headers: { 'Content-Type': 'application/json' },
+      body: JSON.stringify({ name }),
+    });
+    const data = await res.json();
+    return data.module_id || null;
+  } catch (e) {
+    console.error('[api] create-loopback failed:', e);
+    return null;
+  }
+}
+
+export async function unloadModule(moduleId: number) {
+  try {
+    await fetch('/api/unload-module', {
+      method: 'POST',
+      headers: { 'Content-Type': 'application/json' },
+      body: JSON.stringify({ module_id: moduleId }),
+    });
+  } catch (e) {
+    console.error('[api] unload-module failed:', e);
+  }
+}
+
 export { connectPorts, disconnectPorts };