a11y(serious): Course home headers nest buttons

Where: course home outline (${APPS}/learning/course/{course_key}/home), the
six collapsible section headers.

Finding: axe nested-interactive (serious, WCAG 2.2 AA) fails on all six:

target: li:nth-child(1) > .pgn_collapsible > .collapsible-trigger[role="button"]
html:   <div class="collapsible-trigger" role="button" tabindex="0" aria-expanded="false">
why:    Element has focusable descendants

A role="button" element is announced as a single control, so a screen-reader or
keyboard user is told "button" and then meets focusable content inside it that the
role does not account for — the inner controls are not reliably reachable, and the
outer control's purpose is ambiguous.

Suggested fix: make the header's clickable area a real <button> containing
only its label, with the section's links and controls as siblings rather than
descendants as Paragon does.

Start at the course home outline route, ${APPS}/learning/course/{course_key}/home, and locate the six collapsible section headers. Reproduce the axe nested-interactive finding, then verify that the header control no longer contains the section's links or controls and that axe passes for all six headers.