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/vite.config.ts

@@ -0,0 +1,23 @@
+/// <reference types="vitest" />
+import { defineConfig } from 'vitest/config';
+import react from '@vitejs/plugin-react';
+import type { UserConfig } from 'vite';
+
+export default defineConfig({
+  plugins: [react()],
+  server: {
+    port: 5173,
+    strictPort: true,
+  },
+  build: {
+    outDir: 'dist',
+    emptyOutDir: true,
+    base: './',
+  } as UserConfig['build'],
+  test: {
+    environment: 'jsdom',
+    globals: true,
+    setupFiles: ['./src/test/setup.ts'],
+    css: false,
+  },
+});