Applying central Codex design tokens in new architecture.
This replaces all existing MN `z-index` tokens and comments on
replacement choice.
Bug: T285592
Change-Id: I23d794566898946a500c10713802d8dfaad993d1
Content using `header` as a CSS class was affected by skin styles
in Minerva being insufficiently specific to the skin‑specific elements.
This changes Minerva to use `class="minerva‑header"`, thus avoiding
the conflict and matching what Vector does.
Bug: T172626
Change-Id: Id8fbe61b2d1d4a89ec11ddfdf7837be797b3bd20
