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/package.json

@@ -0,0 +1,27 @@
+{
+  "name": "j621-desktop",
+  "productName": "J621",
+  "version": "0.1.0",
+  "private": true,
+  "description": "Desktop shell for the J621 self-hosted media archive",
+  "author": "JakeBreath",
+  "license": "SEE LICENSE IN ../LICENSE",
+  "main": "dist/main.js",
+  "scripts": {
+    "build:main": "tsc -p tsconfig.json",
+    "build:renderer": "npm --prefix ../frontend run build",
+    "build": "npm run build:main && npm run build:renderer",
+    "dev": "npm run build:main && J621_DEV_SERVER=http://localhost:5173 electron .",
+    "start": "npm run build && electron .",
+    "smoke": "npm run build && xvfb-run -a electron . --j621-smoke",
+    "dist:linux": "npm run build && electron-builder --linux",
+    "dist:win": "npm run build && electron-builder --win",
+    "dist:all": "npm run build && electron-builder --linux --win"
+  },
+  "devDependencies": {
+    "@types/node": "^24.13.6",
+    "electron": "^44.4.3",
+    "electron-builder": "^26.15.3",
+    "typescript": "^6.0.3"
+  }
+}