[client] persist Rosenpass static keypair across restarts

The Rosenpass static keypair was regenerated on every engine start
(rp.GenerateKeyPair in NewManager), so the local ~512KB public key —
and its fingerprint — changed on each client restart.

Persist the keypair to <StateDir>/rosenpass_key.json with 0600
permissions (same protection tier as the WireGuard private key), and
reload it on start so the public key stays stable across restarts. A
missing, corrupt, or version-incompatible file degrades gracefully to
generating a fresh ephemeral keypair (previous behaviour); an empty
StateDir keeps the ephemeral path for callers without a state dir.

This is the foundation for fingerprint-based RP pubkey caching over
signalling (NET-1407): a stable local key lets remote peers keep their
cached copy valid across our restart.

client/internal/engine.go

@@ -551,7 +551,7 @@ func (e *Engine) Start(netbirdConfig *mgmProto.NetbirdConfig, mgmtURL *url.URL)
 		} else {
 			log.Infof("running rosenpass in strict mode")
 		}
-		e.rpManager, err = rosenpass.NewManager(e.config.PreSharedKey, e.config.WgIfaceName, publicKey)
+		e.rpManager, err = rosenpass.NewManager(e.config.PreSharedKey, e.config.WgIfaceName, publicKey, e.config.StateDir)
 		if err != nil {
 			return fmt.Errorf("create rosenpass manager: %w", err)
 		}