Add OS keychain for persistent nsec sessions (roadmap #1)

- Rust: store_nsec / load_nsec / delete_nsec Tauri commands via keyring crate
  (macOS Keychain, Windows Credential Manager, Linux Secret Service)
- On nsec login: key is stored in OS keychain keyed by hex pubkey
- On startup: restoreSession() auto-loads nsec from keychain and re-establishes
  the NDK signer — no manual re-login required after restart
- On logout: keychain entry is deleted
- Graceful degradation: if keychain is unavailable (e.g. Linux without a Secret
  Service daemon), the app starts logged-out — same UX as before, no crash

Also updates ROADMAP.md with 4 new items from the Windows playtest (multi-account
switcher, NWC wizard, system tray, zap history view) and reorders the list.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src-tauri/Cargo.toml

@@ -22,4 +22,5 @@ tauri = { version = "2", features = ["devtools"] }
 tauri-plugin-opener = "2"
 serde = { version = "1", features = ["derive"] }
 serde_json = "1"
+keyring = "3"