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

.gitignore

@@ -0,0 +1,14 @@
+# Rust
+/target
+
+# Sensitive local data
+profiles_vault.json
+profiles_vault.json.backup-*
+*.json.tmp
+
+# Frontend
+frontend/node_modules/
+frontend/dist/
+frontend/dist-electron/
+frontend/release/
+frontend/.vite/