feat: Electron desktop app — hardening, IPC bridge, and dev launch fixes

- Dev/prod URL auto-detection (Vite 5173 in dev, server 3000 in prod)
- Typed IPC bridge via preload (notifications, badge, window controls, updates, deep links)
- Native OS notifications via NotificationController with window focus suppression
- Auto-update via electron-updater with UpdateToast UI
- Deep linking (backspace:// protocol) for macOS and Windows/Linux
- Window state persistence (position, size, maximize across restarts)
- Tray icon with graceful fallback when icon asset missing
- Suppress PWA service worker polling/reloads inside Electron
- Platform detection layer (isElectron, getElectronAPI)
- Root workspace scripts (dev:desktop, build:desktop)
- Document BACKSPACE_URL and BACKSPACE_UPDATE_URL env vars

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

@@ -1,10 +1,13 @@
 import { useRegisterSW } from 'virtual:pwa-register/react';
 import { useEffect } from 'react';
+import { isElectron } from '../../platform/platform';
 
 export function SwAutoUpdate() {
+  const inElectron = isElectron();
+
   useRegisterSW({
     onRegisteredSW(_swUrl, registration) {
-      if (!registration) return;
+      if (!registration || inElectron) return;
       setInterval(() => {
         registration.update();
       }, 60_000);
@@ -12,11 +15,12 @@ export function SwAutoUpdate() {
   });
 
   useEffect(() => {
+    if (inElectron) return;
     if (!navigator.serviceWorker) return;
     const onControllerChange = () => window.location.reload();
     navigator.serviceWorker.addEventListener('controllerchange', onControllerChange);
     return () => navigator.serviceWorker.removeEventListener('controllerchange', onControllerChange);
-  }, []);
+  }, [inElectron]);
 
   return null;
 }

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

@@ -0,0 +1,52 @@
+import React, { useState, useEffect } from 'react';
+import { isElectron } from '../../platform/platform';
+
+/**
+ * Persistent toast shown when an Electron auto-update has been downloaded.
+ * Renders nothing in browser environments.
+ */
+export function UpdateToast() {
+  const [downloadedVersion, setDownloadedVersion] = useState<string | null>(null);
+
+  useEffect(() => {
+    if (!isElectron() || !window.backspace) return;
+
+    window.backspace.onUpdateDownloaded((info) => {
+      setDownloadedVersion(info.version);
+    });
+  }, []);
+
+  if (!downloadedVersion) return null;
+
+  const handleRestart = () => {
+    window.backspace?.installUpdate();
+  };
+
+  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={handleRestart}
+          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>
+  );
+}