fix: IDEAS.md quickfixes — idle-timeout app config, id_token out of Claims, FormEvent import

session.idle_timeout (minutes, default 15) joins the app-config registry
and drives the onboarding Garmin session eviction, distinct from
session.duration (the login cookie lifetime in hours). The raw Keycloak
ID token no longer rides in auth.Claims through every request context:
it's minted into the session cookie separately and read back only by the
logout handler via IDTokenFromSessionCookie. OnboardingWizard uses the
type-imported FormEvent<HTMLFormElement> instead of the React.FormEvent
namespace alias.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

backend/internal/api/server.go

@@ -53,6 +53,11 @@ type Server struct {
 	// never call os.Getenv.
 	EnvVars []EnvVar
 
+	// SetupSessionIdleTimeout is the app-config session.idle_timeout value
+	// (see internal/config); zero falls back to
+	// defaultSetupSessionIdleTimeout.
+	SetupSessionIdleTimeout time.Duration
+
 	mu              sync.Mutex
 	userClient      map[int64]garmin.Client
 	userSync        map[int64]*garmin.Sync