feat: pactl-based module loading, more device types

Backend:
- Switched null-sink and loopback to use pactl (works on all systems)
- Generic POST /api/load-module {module, args} for any pactl module
- Fixed SVG toolbar click issue (z-index layering)

Frontend:
- + Add Device dropdown now includes:
  - Null Sink (virtual audio output)
  - Loopback Device (paired input+output)
  - TCP Network Server (module-native-protocol-tcp)
  - TCP Tunnel Sink
  - TCP Tunnel Source
- Dropdown header and section separators
- Fixed canvas z-index so toolbar is clickable

frontend/src/lib/stores.ts

@@ -392,7 +392,7 @@ export async function createNullSink(name: string): Promise<number | null> {
       body: JSON.stringify({ name }),
     });
     const data = await res.json();
-    return data.module_id || null;
+    return data.module_id > 0 ? data.module_id : null;
   } catch (e) {
     console.error('[api] create-null-sink failed:', e);
     return null;
@@ -407,13 +407,28 @@ export async function createLoopback(name: string): Promise<number | null> {
       body: JSON.stringify({ name }),
     });
     const data = await res.json();
-    return data.module_id || null;
+    return data.module_id > 0 ? data.module_id : null;
   } catch (e) {
     console.error('[api] create-loopback failed:', e);
     return null;
   }
 }
 
+export async function loadModule(module: string, args: string): Promise<number | null> {
+  try {
+    const res = await fetch('/api/load-module', {
+      method: 'POST',
+      headers: { 'Content-Type': 'application/json' },
+      body: JSON.stringify({ module, args }),
+    });
+    const data = await res.json();
+    return data.module_id > 0 ? data.module_id : null;
+  } catch (e) {
+    console.error('[api] load-module failed:', e);
+    return null;
+  }
+}
+
 export async function unloadModule(moduleId: number) {
   try {
     await fetch('/api/unload-module', {