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/README.md

@@ -0,0 +1,66 @@
+# J621 desktop (Electron)
+
+An Electron shell around the web build. The main process serves the SPA from
+the privileged `app://j621` scheme — a real origin, so `localStorage`, OPFS,
+Web Workers and WebCodecs all behave exactly like they do in Chrome — and the
+SPA keeps its normal first-run flow: on launch it asks where the backend is
+(or runs in local mode with no backend at all).
+
+The renderer code is not forked: the shell loads `frontend/dist` as built by
+`npm run build` in `frontend/`.
+
+## Layout
+
+```
+src/main.ts      window, app:// protocol handler, menu, external links
+src/preload.ts   window.j621Desktop bridge
+electron-builder.yml   deb + pacman + nsis packaging
+```
+
+## Development
+
+```bash
+# terminal 1: the SPA with its /api proxy (needs the backend for library pages)
+cd frontend && npm run dev
+
+# terminal 2
+cd desktop && npm run dev
+```
+
+`npm run dev` builds the main process and points the window at
+`http://localhost:5173` (`J621_DEV_SERVER`). Without the dev server,
+`npm start` builds both halves and serves the bundled SPA over `app://j621`.
+
+## Builds
+
+```bash
+npm run dist:linux   # release/*.deb + release/*.pkg.tar.zst
+npm run dist:win     # release/J621-Setup-*.exe (cross-built with wine; untested)
+npm run dist:all
+```
+
+The Arch package can be installed and removed with pacman:
+
+```bash
+sudo pacman -U release/j621-*.pkg.tar.zst
+```
+
+## Smoke test
+
+`npm run smoke` builds everything, runs Electron headless through `xvfb-run`
+and checks that the bundled SPA loads over `app://j621`: the SPA fallback, an
+asset fetch, `localStorage`, OPFS, WebCodecs and the preload bridge. It exits
+non-zero on failure.
+
+## Backend CORS
+
+A desktop app is cross-origin to the backend, exactly like the frontend-only
+Docker deployment. Add the shell's origin to the backend:
+
+```
+CORS_ALLOWED_ORIGINS=app://j621
+```
+
+The setup screen shows this hint (with the actual origin) when the connection
+test fails. Without a backend, choose "Continue without a backend" — the
+e621-facing pages work the same as in the browser.