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/electron/preload.ts

@@ -0,0 +1,7 @@
+import { contextBridge, ipcRenderer } from 'electron';
+
+contextBridge.exposeInMainWorld('backend', {
+  request: (method: string, params?: Record<string, unknown>): Promise<unknown> =>
+    ipcRenderer.invoke('backend:request', { method, params }),
+  copyText: (text: string): Promise<void> => ipcRenderer.invoke('clipboard:write', text),
+});