feat(sidebar): opt-in ARIA landmark roles; aria-controls on nav links

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Stacked on #2523

This PR ports the R-side bslib changes that became necessary after #2523 bumped the vendored bslib assets from 97aa1abc to ac1e25f8. Its base is fix-input-date-datesdisabled-format (a copy of #2523's branch pushed to this repo), so the diff shows only this PR's commits. Once #2523 merges to main, delete the copy branch and GitHub will retarget this PR to main.

An audit of the bslib range found 6 commits; the other 4 needed no Python changes: rstudio/bslib#1349 and rstudio/bslib#1352 are TS/SCSS-only (carried by the vendored assets), rstudio/bslib#1353 is a DESCRIPTION change, and rstudio/bslib#1351 fixes a duplicate-id bug py-shiny never had (its toolbar_input_select() already gives the <select> its own {id}-select id).

Port of rstudio/bslib#1359: sidebar landmark roles

• ui.sidebar() gains a role parameter ("form", "search", "complementary", "region"). The default, None, now renders a neutral <div> instead of an <aside>; "complementary" renders an <aside>, and other roles render a <div> with the corresponding role attribute (e.g. <div role="form">).
• Landmark roles require an accessible name. An explicit aria_label=/aria_labelledby= (passed via **kwargs) always wins and is hoisted from the content div onto the landmark element; otherwise the landmark is labelled by the sidebar's title (via aria-labelledby, generating a title id when needed). Non-tag titles (e.g. HTML()) are wrapped in a display:contents div so they can hold an id. A landmark sidebar with no name raises ValueError.
• ui.page_sidebar() now places the whole layout_sidebar() inside the page's <main class="bslib-page-main"> landmark (previously only the main-content children were), and the <main> no longer gets bslib-gap-spacing (page_main_container() gains a gap parameter).
• playwright.controller.Sidebar now locates the sidebar by class (> .sidebar#{id}) instead of the aside element, since the element is no longer always an <aside>.
• Express ui.sidebar() gains the same role parameter.

Note the vendored page_sidebar.scss (updated in #2523) already expects this new markup.

Port of rstudio/bslib#1355: aria-controls on nav links

Nav links that target a tab panel now carry aria-controls pointing at the panel's id, alongside the existing href.

Testing

• New pytest coverage: neutral/complementary/landmark markup, title-derived and explicit accessible names, non-tag title wrapping, the accessible-name error, invalid roles, the page_sidebar() main-landmark structure, and aria-controls on nav links. Existing nav markup expectations updated.
• tests/pytest: 1226 passed, 6 skipped (the 14 test_app_ui_page_html.py errors are a pre-existing local starlette/httpx environment issue, also present on the base branch).
• flake8 and pyrefly pass on the changed files (make check-lint/make check-types fail locally on pre-existing sandbox files/missing pyrefly, unrelated to this change).