a11y: narrow-viewport sidebar toolbar is a nav landmark with no links

The bar that appears below the navbar under the lg breakpoint is marked up as a
nav, so it becomes a navigation landmark. It holds the sidebar toggle button,
the page title or the breadcrumbs, and the search button. In neither
configuration does the nav itself hold a navigation link.

https://github.com/quarto-dev/quarto-cli/blob/093316c3348715aab8447bd09faa2eca0ca0e98a/src/resources/projects/website/templates/nav-before-body.ejs#L63-L89

• With breadcrumbs on (the default), every link is inside a nested
  nav.quarto-page-breadcrumbs, which is its own landmark. The postprocessor
  swaps it in for the title element:
  https://github.com/quarto-dev/quarto-cli/blob/093316c3348715aab8447bd09faa2eca0ca0e98a/src/project/types/website/website-navigation.ts#L470-L482
• With breadcrumbs: false there are no links at all. The <a> at L74 and L78
  has no href. It is a Bootstrap collapse trigger, and #14826 already removed
  the role="navigation" and role="link" it used to carry.

So the landmark holds a button, a heading, and another landmark.

The strip only exists below lg, and the sidebar it toggles is a collapse that
sits under it, so both are in the accessibility tree together when a reader opens
the sidebar:

https://github.com/quarto-dev/quarto-cli/blob/093316c3348715aab8447bd09faa2eca0ca0e98a/src/resources/projects/website/navigation/quarto-nav.scss#L454-L493

This surfaced in review of #14813, which gives every nav landmark a distinct
aria-label. Naming this one is hard because there is no honest navigation word
for it. The label there is Toolbar, which a screen reader announces as
"Toolbar, navigation".

axe does not flag this. landmark-unique only requires that the labels differ,
which #14813 satisfies. This is a markup question, not a rule violation.

Suggested fix

Change the nav to a div and drop its aria-label. The nested breadcrumbs
nav keeps its own landmark and label, so the only difference for a reader is
that the Toolbar entry drops out of the landmark list. Nothing else moves, and
no links become unreachable.

Points to check:

• Most CSS selectors are class based (.quarto-secondary-nav), but four rules
  use the element selector nav.quarto-secondary-nav and need updating.
• With breadcrumbs: false the bar then holds no landmark at all. Make sure the
  page title inside it is still reachable.
• The navigation-toolbar-label language key added by #14813 becomes unused. It
  ships in _language.yml and in all 33 translated files, so remove it before
  1.11 is released rather than after.

Split from #14813. Part of #8706.

Investigated with AI (Claude Code), grounded in a local clone.

Start with src/resources/projects/website/templates/nav-before-body.ejs and update the four nav.quarto-secondary-nav selectors in src/resources/projects/website/navigation/quarto-nav.scss. Check the breadcrumbs and breadcrumbs: false configurations, then remove navigation-toolbar-label from _language.yml and all 33 translated files. Done means the toolbar is a div, the page title remains reachable, and the nested breadcrumbs landmark is unchanged.