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/web_server.cpp

@@ -446,6 +446,88 @@ void WebServer::setupRoutes() {
 
     m_http.Options("/api/volume", cors_handler);
     m_http.Options("/api/mute", cors_handler);
+    m_http.Options("/api/create-null-sink", cors_handler);
+    m_http.Options("/api/create-loopback", cors_handler);
+    m_http.Options("/api/unload-module", cors_handler);
+
+    // Create null sink: POST /api/create-null-sink {"name":"My Sink"}
+    m_http.Post("/api/create-null-sink", [this](const httplib::Request &req, httplib::Response &res) {
+        // Parse name from JSON - simple extraction
+        std::string name = "pwweb-null";
+        size_t pos = req.body.find("\"name\"");
+        if (pos != std::string::npos) {
+            size_t start = req.body.find('"', pos + 6);
+            if (start != std::string::npos) {
+                start++;
+                size_t end = req.body.find('"', start);
+                if (end != std::string::npos) {
+                    name = req.body.substr(start, end - start);
+                }
+            }
+        }
+
+        std::string args = "node.name=" + name +
+            " media.class=Audio/Sink " +
+            "audio.position=[FL FR]";
+
+        uint32_t id = m_engine.loadModule(
+            "libpipewire-module-null-sink",
+            args.c_str());
+
+        if (id > 0) broadcastGraph();
+
+        char buf[128];
+        snprintf(buf, sizeof(buf), "{\"ok\":%s,\"module_id\":%u}",
+            id > 0 ? "true" : "false", id);
+        res.set_content(buf, "application/json");
+        res.set_header("Access-Control-Allow-Origin", "*");
+    });
+
+    // Create loopback: POST /api/create-loopback {"name":"My Loopback"}
+    m_http.Post("/api/create-loopback", [this](const httplib::Request &req, httplib::Response &res) {
+        std::string name = "pwweb-loopback";
+        size_t pos = req.body.find("\"name\"");
+        if (pos != std::string::npos) {
+            size_t start = req.body.find('"', pos + 6);
+            if (start != std::string::npos) {
+                start++;
+                size_t end = req.body.find('"', start);
+                if (end != std::string::npos) {
+                    name = req.body.substr(start, end - start);
+                }
+            }
+        }
+
+        std::string args = "node.name=" + name +
+            " capture.props=node.name=" + name + "-capture" +
+            " playback.props=node.name=" + name + "-playback";
+
+        uint32_t id = m_engine.loadModule(
+            "libpipewire-module-loopback",
+            args.c_str());
+
+        if (id > 0) broadcastGraph();
+
+        char buf[128];
+        snprintf(buf, sizeof(buf), "{\"ok\":%s,\"module_id\":%u}",
+            id > 0 ? "true" : "false", id);
+        res.set_content(buf, "application/json");
+        res.set_header("Access-Control-Allow-Origin", "*");
+    });
+
+    // Unload module: POST /api/unload-module {"module_id":N}
+    m_http.Post("/api/unload-module", [this](const httplib::Request &req, httplib::Response &res) {
+        uint32_t module_id = 0;
+        if (sscanf(req.body.c_str(), "{\"module_id\":%u}", &module_id) == 1) {
+            bool ok = m_engine.unloadModule(module_id);
+            if (ok) broadcastGraph();
+            res.set_content(ok ? "{\"ok\":true}" : "{\"ok\":false}", "application/json");
+        } else {
+            res.status = 400;
+            res.set_content("{\"error\":\"invalid json\"}", "application/json");
+        }
+        res.set_header("Access-Control-Allow-Origin", "*");
+    });
 }
 
 // end of web_server.cpp