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.

frontend/src/features/setup/SetupPage.tsx

@@ -41,11 +41,17 @@ async function testBackend(base: string): Promise<TestResult> {
       text: `The backend answered HTTP ${response.status}.`,
     };
   } catch {
+    // The desktop shell is a different origin from the backend, and that is
+    // the usual reason the first connection test fails there.
+    const desktopHint = window.j621Desktop
+      ? ` The desktop app's origin is ${window.j621Desktop.origin} — allow it in CORS_ALLOWED_ORIGINS on the backend.`
+      : "";
     return {
       ok: false,
       text:
         "Could not reach the backend. Check the URL, and if it lives on " +
-        "another domain add this site to CORS_ALLOWED_ORIGINS there.",
+        "another domain add this site to CORS_ALLOWED_ORIGINS there." +
+        desktopHint,
     };
   } finally {
     window.clearTimeout(timeout);