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

- Changed Container default bottom spacing from padding-bottom to margin-bottom
- Prevented the Container margin-bottom from applying when used as the Header root element via mb="0"
- Renamed Header CSS classes from bui-HeaderPage* to bui-Header* to match the component name

Fixes https://linear.app/spotify/issue/BACUI-249

Signed-off-by: Charles de Dreuille <charles.dedreuille@gmail.com>
Made-with: Cursor

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

@@ -0,0 +1,5 @@
+---
+'@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.

.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 notes**: 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