[client] Extract SSH JWT cache into shared jwtcache package

Move the daemon's memguard-backed JWT token cache from client/server
into client/ssh/jwtcache so it can be reused outside the daemon
process. The TTL derivation from Config.SSHJWTCacheTTL (nil=default,
0=disabled) moves into jwtcache.ResolveTTL.

Wire the cache into the iOS SDK's SSH client: iOS has no daemon to
delegate caching to, so without it every reconnect forced the user
through the browser OAuth device-code flow. The cache lives on the
long-lived Client (the app creates a new SSHClient per session) and
uses the same config-driven TTL semantics as the daemon.

client/ios/NetBirdSDK/client.go

@@ -21,6 +21,7 @@ import (
 	"github.com/netbirdio/netbird/client/internal/listener"
 	"github.com/netbirdio/netbird/client/internal/peer"
 	"github.com/netbirdio/netbird/client/internal/profilemanager"
+	"github.com/netbirdio/netbird/client/ssh/jwtcache"
 	"github.com/netbirdio/netbird/client/system"
 	"github.com/netbirdio/netbird/formatter"
 	"github.com/netbirdio/netbird/route"
@@ -87,6 +88,11 @@ type Client struct {
 	// config holds the active configuration once Run has loaded it. Consumed by
 	// the in-app SSH client for the NetBird SSH key and the OAuth flow.
 	config *profilemanager.Config
+
+	// sshJWTCache keeps the SSH JWT token between reconnects so the user is not
+	// forced through the browser OAuth flow on every session. Lives on Client
+	// (not SSHClient) because the app creates a new SSHClient per session.
+	sshJWTCache *jwtcache.Cache
 }
 
 // NewClient instantiate a new Client
@@ -103,6 +109,7 @@ func NewClient(cfgFile, stateFile, cacheDir, logFilePath, deviceName string, osV
 		ctxCancelLock:         &sync.Mutex{},
 		networkChangeListener: networkChangeListener,
 		dnsManager:            dnsManager,
+		sshJWTCache:           jwtcache.New(),
 	}
 }