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

Cargo.toml

@@ -0,0 +1,12 @@
+[package]
+name = "nostr-manager-backend"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
+nostr-sdk = { version = "0.40", features = ["nip44"] }
+tokio = { version = "1", features = ["full"] }
+serde = { version = "1.0", features = ["derive"] }
+serde_json = "1.0"
+uuid = { version = "1.0", features = ["v4"] }
+hex = "0.4"