panel-redesign: sign-in without an administrator follows its mockup

- panel.css: .sp-signin-form .sp-form, as theme.css has it now.
  - templates/login.html: the state is div.sp-form with the mockup's
    heading and line; the empty form of ef8a82e is gone.
  - tests: the hint is checked by its new wording, and that the page
    carries no form at all.

Evidence
  - docs/assets/panel-redesign/evidence/login-no-admin.png: 1440x900,
    headless Edge, real templates served locally (no Docker here).
    login.png re-shot after the selector change: byte-identical.
  - go test -count=1 ./internal/web/... : ok (web, auth, handlers,
    validate, view)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/web/auth/handlers_test.go

@@ -133,7 +133,7 @@ func TestLoginPointsAtSetupBeforeTheFirstAdministrator(t *testing.T) {
 		t.Fatalf("status = %d, want 200", rec.Code)
 	}
 	body := rec.Body.String()
-	if !strings.Contains(body, "No administrator has been created yet") {
+	if !strings.Contains(body, "No administrator yet") || !strings.Contains(body, "one-time setup link") {
 		t.Errorf("the login page does not point at the setup link:\n%s", body)
 	}
 	if strings.Contains(body, `name="password"`) {