Merge pull request #33354 from backstage/charlesdedreuille/bacui-249-fix-container-bottom-margin-clash-in-bui-oss

fix(ui): fix Container bottom margin clash in Header

.changeset/fix-header-container-padding.md

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+---
+'@backstage/ui': patch
+---
+
+Fixed incorrect bottom spacing caused by `Container` using `padding-bottom` for its default bottom spacing. Changed to `margin-bottom` and prevented it from applying when `Container` is used as the `Header` root element.
+
+**Affected components:** Container, Header

.changeset/rename-header-css-classes.md

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+---
+'@backstage/ui': minor
+---
+
+**BREAKING**: Renamed internal CSS classes to match the `Header` component name.
+
+**Migration:**: If you are targeting these classes directly in your styles, update the following:
+
+- `bui-HeaderPage` → `bui-Header`
+- `bui-HeaderPageContent` → `bui-HeaderContent`
+- `bui-HeaderPageBreadcrumbs` → `bui-HeaderBreadcrumbs`
+- `bui-HeaderPageTabsWrapper` → `bui-HeaderTabsWrapper`
+- `bui-HeaderPageControls` → `bui-HeaderControls`
+
+**Affected components:**: Header