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.

deploy/.env.example

@@ -31,6 +31,9 @@ ALLOWED_HOSTS=j621.rainbow-herring.ts.net,localhost,127.0.0.1
 # Cross-origin access (needed for the separate frontend + backend deploys)
 # ---------------------------------------------------------------------------
 # The origin the SPA is served from, e.g. https://j621-frontend.<tailnet>.ts.net
+# The desktop app (desktop/) is served from app://j621, so add that origin too
+# when it should reach this backend:
+#   CORS_ALLOWED_ORIGINS=https://j621-frontend.<tailnet>.ts.net,app://j621
 # CORS_ALLOWED_ORIGINS=
 # CSRF_TRUSTED_ORIGINS=