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/electron-builder.yml

@@ -0,0 +1,42 @@
+appId: io.j621.desktop
+productName: J621
+copyright: Copyright (c) 2026 JakeBreath — Jake Labs Non-Commercial Software Licence
+
+directories:
+  output: release
+  buildResources: build
+
+# The Electron main process and preload live in dist/ (tsc output). The SPA is
+# copied from the web build as an extra resource and served over app://j621.
+files:
+  - dist/**
+  - package.json
+
+extraResources:
+  - from: ../frontend/dist
+    to: dist
+    filter:
+      - "**/*"
+
+linux:
+  target:
+    - deb
+    - pacman
+  category: AudioVideo
+  synopsis: Desktop client for the J621 self-hosted media archive
+  description: >-
+    J621 is a self-hosted browser for an e621-linked media library. This
+    desktop build talks to a J621 backend over the network, the same way the
+    web app does.
+  icon: build/icon.png
+
+win:
+  target:
+    - nsis
+  icon: build/icon.ico
+
+nsis:
+  oneClick: false
+  perMachine: false
+  allowToChangeInstallationDirectory: true
+  shortcutName: J621