feat(desktop): add manual download fallback to UpdateToast

Extends UpdateToast to handle update-error events from the preload bridge,
showing a "Download" link to the GitHub release URL when auto-update fails.

packages/web/src/components/ui/UpdateToast.tsx

@@ -1,44 +1,88 @@
-import React, { useState, useEffect } from 'react';
+import { useState, useEffect } from 'react';
 import { isElectron } from '../../platform/platform';
 
+interface UpdateError {
+  message: string;
+  releaseUrl: string;
+}
+
 /**
- * Persistent toast shown when an Electron auto-update has been downloaded.
+ * Persistent toast shown when an Electron auto-update has been downloaded
+ * or when auto-update fails (offers manual download link).
  * Renders nothing in browser environments.
  */
 export function UpdateToast() {
   const [downloadedVersion, setDownloadedVersion] = useState<string | null>(null);
+  const [failedUpdate, setFailedUpdate] = useState<UpdateError | null>(null);
 
   useEffect(() => {
     if (!isElectron() || !window.backspace) return;
 
     window.backspace.onUpdateDownloaded((info) => {
       setDownloadedVersion(info.version);
+      // Auto-download succeeded — clear any previous error state
+      setFailedUpdate(null);
+    });
+
+    window.backspace.onUpdateError((error) => {
+      setFailedUpdate(error);
     });
   }, []);
 
-  if (!downloadedVersion) return null;
+  // Nothing to show
+  if (!downloadedVersion && !failedUpdate) return null;
 
-  const handleRestart = () => {
+  // Auto-download succeeded — show restart toast
-    window.backspace?.installUpdate();
+  if (downloadedVersion) {
-  };
+    return (
+      <div className="fixed bottom-6 left-6 z-[300] animate-slide-up">
+        <div className="glass-pill rounded-xl px-4 py-3 flex items-center gap-3 max-w-[340px]">
+          <div className="flex-1 min-w-0">
+            <p className="text-sm font-medium text-txt-primary">Update ready</p>
+            <p className="text-xs text-txt-secondary truncate">
+              Version {downloadedVersion} has been downloaded
+            </p>
+          </div>
+          <button
+            onClick={() => window.backspace?.installUpdate()}
+            className="shrink-0 px-3 py-1.5 text-xs font-medium rounded-lg bg-accent-primary hover:bg-accent-primary/80 text-white transition-colors"
+          >
+            Restart
+          </button>
+          <button
+            onClick={() => setDownloadedVersion(null)}
+            className="shrink-0 p-1 text-txt-tertiary hover:text-txt-secondary transition-colors"
+            aria-label="Dismiss"
+          >
+            <svg className="w-4 h-4" fill="none" viewBox="0 0 24 24" stroke="currentColor" strokeWidth={2}>
+              <path strokeLinecap="round" strokeLinejoin="round" d="M6 18L18 6M6 6l12 12" />
+            </svg>
+          </button>
+        </div>
+      </div>
+    );
+  }
 
+  // Auto-download failed — show manual download toast
   return (
     <div className="fixed bottom-6 left-6 z-[300] animate-slide-up">
-      <div className="glass-pill rounded-xl px-4 py-3 flex items-center gap-3 max-w-[340px]">
+      <div className="glass-pill rounded-xl px-4 py-3 flex items-center gap-3 max-w-[380px]">
         <div className="flex-1 min-w-0">
-          <p className="text-sm font-medium text-txt-primary">Update ready</p>
+          <p className="text-sm font-medium text-txt-primary">Update available</p>
           <p className="text-xs text-txt-secondary truncate">
-            Version {downloadedVersion} has been downloaded
+            Auto-install failed — download manually
           </p>
         </div>
-        <button
+        <a
-          onClick={handleRestart}
+          href={failedUpdate!.releaseUrl}
+          target="_blank"
+          rel="noopener noreferrer"
           className="shrink-0 px-3 py-1.5 text-xs font-medium rounded-lg bg-accent-primary hover:bg-accent-primary/80 text-white transition-colors"
         >
-          Restart
+          Download
-        </button>
+        </a>
         <button
-          onClick={() => setDownloadedVersion(null)}
+          onClick={() => setFailedUpdate(null)}
           className="shrink-0 p-1 text-txt-tertiary hover:text-txt-secondary transition-colors"
           aria-label="Dismiss"
         >