Add banner and main landmarks to page_navbar()

[jcheng5]

with @shelbylevel, and Posit Asisstant/Kimi K3

Fixes #1356

Wrap the top navigation bar in a <header> element so the page has a banner landmark, and render the content area as <main> when no sidebar is present (sidebar pages already get one via page_main_container()).

Both changes are made in page_navbar() rather than the shared navs_bar_() so that embedded navset_bar() instances don't produce inappropriate page-level landmarks. The <main> change is done in-place (the content div's tag is renamed) to preserve fill layout behavior.

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