* admin: fix mobile sidebar menu inaccessible in portrait mode
The hamburger button only toggled the user dropdown, leaving the
sidebar navigation inaccessible on mobile devices in portrait mode.
Add a dedicated sidebar toggle button (visible only on mobile), give
the sidebar an id so Bootstrap collapse can target it, add a backdrop
overlay for the open state, and auto-close the sidebar when a nav
link is clicked.
Fixes#8550
* admin: address review feedback on mobile sidebar
- Remove redundant JS show/hide.bs.collapse listeners; CSS sibling
selector already handles backdrop visibility
- Use const instead of var for non-reassigned variables
- Move inline style on user icon to CSS class
* admin: add aria attributes to user-menu toggler, use CSS variable for navbar height
- Add aria-controls, aria-expanded, and aria-label to the user-menu
toggle button for assistive technology
- Extract hard-coded 56px navbar height into --navbar-height CSS
custom property used by sidebar and backdrop positioning
* admin: extract hideSidebar helper, use toggler visibility for breakpoint check
- Extract duplicated collapse-hide logic into a hideSidebar helper
- Replace hardcoded window.innerWidth < 768 with a check on the
sidebar toggler's computed display, decoupling JS from CSS breakpoints
- Add aria-expanded="false" to sidebar toggle button
---------
Co-authored-by: Copilot <copilot@github.com>
e25558e4d8