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/.prettierrc

@@ -0,0 +1,6 @@
+{
+  "singleQuote": true,
+  "printWidth": 100,
+  "trailingComma": "all",
+  "semi": true
+}