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/isolation_test.go

@@ -31,7 +31,7 @@ func doJSONAs(t *testing.T, handler http.Handler, sub, method, path string, body
 	}
 	req := httptest.NewRequest(method, path, reader)
 	req.Header.Set("Content-Type", "application/json")
-	cookie, err := auth.MintSessionCookie(auth.Claims{Sub: sub, Name: sub, Email: sub + "@example.com"}, testSessionConfig.Secret, testSessionConfig.Duration, testSessionConfig.Secure)
+	cookie, err := auth.MintSessionCookie(auth.Claims{Sub: sub, Name: sub, Email: sub + "@example.com"}, "", testSessionConfig.Secret, testSessionConfig.Duration, testSessionConfig.Secure)
 	if err != nil {
 		t.Fatalf("mint test session cookie: %v", err)
 	}