Fix PluginHeader ResizeObserver loop

Defer header height updates to avoid ResizeObserver loop warnings in FullPage layouts and normalize header actions before rendering.

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>
Made-with: Cursor

.changeset/fix-plugin-header-resizeobserver-loop.md

@@ -0,0 +1,7 @@
+---
+'@backstage/ui': patch
+---
+
+Fixed `PluginHeader` to avoid triggering `ResizeObserver loop completed with undelivered notifications` warnings when used in layouts that react to the header height, such as pages that use `FullPage`.
+
+**Affected components:** PluginHeader