fix: resolve OpenVINO NPU and GPU backend switching/loading transition issues

app/frontend/src/components/Generator.jsx

@@ -139,7 +139,7 @@ function Generator({
             crashError = status.error;
             break;
           }
-          if (!status.running && i > 3) {
+          if (!status.running && !status.loading?.active && i > 3) {
             crashError = "The backend process terminated immediately on startup.";
             break;
           }

app/frontend/src/components/ImageConstraints.jsx

@@ -86,18 +86,17 @@ function ImageConstraints({
     const currentBackend = constraints.backendType || "cpu";
     if (backendType === currentBackend) return;
 
-    const isGpuBackend = (type) => !["cpu", "openvino-npu"].includes(type);
     const switchesAccelerator =
-      (currentBackend === "openvino-npu" && isGpuBackend(backendType)) ||
-      (isGpuBackend(currentBackend) && backendType === "openvino-npu");
+      (currentBackend === "openvino-npu" && backendType !== "openvino-npu") ||
+      (currentBackend !== "openvino-npu" && backendType === "openvino-npu");
 
     if (serverRunning && switchesAccelerator) {
       const leavingNpu = currentBackend === "openvino-npu";
       const confirmed = await showConfirm({
-        title: leavingNpu ? "Unload NPU Model?" : "Unload GPU Model?",
+        title: leavingNpu ? "Unload NPU Model?" : "Unload Model?",
         message: leavingNpu
-          ? "The OpenVINO NPU model must be unloaded before switching to the GPU backend."
-          : "The GPU model must be unloaded before switching to the OpenVINO NPU backend.",
+          ? "The OpenVINO NPU model must be unloaded before switching to the standard backend."
+          : "The active model must be unloaded before switching to the OpenVINO NPU backend.",
         confirmLabel: "Unload",
         cancelLabel: "Cancel",
         danger: true,

app/frontend/src/components/ModelManager.jsx

@@ -332,7 +332,9 @@ function ModelManager({ activeModel, setActiveModel, serverRunning, setServerRun
       
       let isReady = false;
       let crashError = null;
-      for (let i = 0; i < 240; i++) { // Poll for up to 120 seconds (240 * 500ms)
+      const isOpenVinoModel = modelInfo?.backendType === "openvino-npu";
+      const maxStartupPolls = isOpenVinoModel ? 1200 : 240;
+      for (let i = 0; i < maxStartupPolls; i++) {
         const status = await getBackendStatus();
         if (status.loading) {
           setModelLoadProgress({
@@ -355,7 +357,7 @@ function ModelManager({ activeModel, setActiveModel, serverRunning, setServerRun
           crashError = status.error;
           break;
         }
-        if (!status.running && i > 3) {
+        if (!status.running && !status.loading?.active && i > 3) {
           crashError = "The backend process terminated immediately on startup.";
           break;
         }