fix(core): pin CorePageHeader to content size in flex-column contexts (#4210)

v-row inherits flex: 1 1 auto from Vuetify. Add flex: 0 0 auto on
.core-page-header so the header never grows when placed as a flex child
of a flex-direction:column shell. No-op in normal block flow.

Closes #4202

Author: PierreBrisorgueil

src/modules/core/components/core.pageHeader.component.vue

@@ -85,6 +85,10 @@ export default {
  * a 56px rhythm for title↔breadcrumb route transitions use CorePageHeaderTabs,
  * which enforces the height externally.
  */
+.core-page-header {
+  flex: 0 0 auto; /* never grow on the main axis of a flex-column parent — v-row inherits flex: 1 1 auto from Vuetify */
+}
+
 .core-page-header__content {
   flex: 1 1 0;
   min-width: 0;