Add Nostr Feed Manager: Rust backend with Electron + React GUI

- Rust library (nostr-manager-backend) with CLI and JSON-lines IPC serve mode:
  profiles, publishing with per-relay reports, relays, settings, vault storage
  and legacy-vault migration
- Electron + React + TypeScript desktop GUI using the same backend over stdio IPC
- Vitest suite with a fake backend speaking the real protocol
- electron-builder linux packaging; README with build and usage instructions

frontend/src/main.tsx

@@ -0,0 +1,15 @@
+import { StrictMode } from 'react';
+import { createRoot } from 'react-dom/client';
+import App from './App';
+import './styles.css';
+
+const rootElement = document.getElementById('root');
+if (!rootElement) {
+  throw new Error('Root element #root not found');
+}
+
+createRoot(rootElement).render(
+  <StrictMode>
+    <App />
+  </StrictMode>,
+);