Add an Electron desktop shell for the SPA

desktop/ serves the normal frontend build over a privileged app://j621
scheme, so localStorage, OPFS, Web Workers, WebCodecs and history routing
behave exactly like Chrome. Development points at the Vite dev server;
`npm run smoke` runs headless Electron and checks the bundled app.

External links open in the system browser, and download navigations
(?download=1 or media URLs) are rerouted through webContents.downloadURL,
since preventing them cancels the download. The setup screen names the
shell's origin when the connection test fails, and the deploy docs list
app://j621 for CORS_ALLOWED_ORIGINS.

desktop/src/preload.ts

@@ -0,0 +1,27 @@
+/**
+ * The only bridge between the desktop shell and the SPA. Keep it tiny: the
+ * renderer is the normal web build and must not need Electron to work.
+ */
+import { contextBridge, ipcRenderer } from "electron";
+
+contextBridge.exposeInMainWorld("j621Desktop", {
+  isDesktop: true as const,
+  platform: process.platform,
+  // The SPA's own origin (`app://j621`), shown by the setup screen when it
+  // has to explain which origin to allow in the backend's CORS settings.
+  origin: window.location.origin,
+  getVersion: (): Promise<string> => ipcRenderer.invoke("j621:version"),
+});
+
+// Menu → "Open backend in browser": read the SPA's stored backend URL and
+// hand it to the main process after checking it is a real web URL.
+ipcRenderer.on("j621:open-backend", () => {
+  let url = "";
+  try {
+    url = window.localStorage.getItem("j621.backend") ?? "";
+  } catch {
+    // Storage can be unavailable in rare cases; nothing to open then.
+  }
+  if (url === "none") url = "";
+  if (url) void ipcRenderer.invoke("j621:open-external", url);
+});