feat(a11y): remediate WCAG 2.2 AA and EAA conformance baseline

- Standardized SidebarInset.vue wrapper to a semantic <div> to prevent nested/duplicate <main> landmarks.
- Disabled Vue DevTools in E2E environments using the PLAYWRIGHT=true env mapping to prevent injected non-conforming HTML elements from failing automated scans.
- Added explicit tabindex="-1" to CreatePostModal.vue <dialog> to support reliable focus trapping.
- Added missing aria-labels to navigation and media upload icon buttons in the scheduler and composer views.
- Upgraded the E2E accessibility suite to support sequential login form tab-traversal, direct canonical /scheduler/calendar/week navigation, and robust toPass assertion testing for focus trap.

Co-authored-by: yacosta738 <33158051+yacosta738@users.noreply.github.com>

Author: google-labs-jules[bot]