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/tsconfig.json

@@ -0,0 +1,19 @@
+{
+  "compilerOptions": {
+    "target": "ES2022",
+    "module": "node16",
+    "moduleResolution": "node16",
+    "lib": ["ES2023", "DOM"],
+    "types": ["node"],
+    "outDir": "dist",
+    "rootDir": "src",
+    "sourceMap": true,
+    "strict": true,
+    "noUnusedLocals": true,
+    "noUnusedParameters": true,
+    "esModuleInterop": true,
+    "skipLibCheck": true,
+    "forceConsistentCasingInFileNames": true
+  },
+  "include": ["src"]
+}