a11y (critical) - grading grade-range editor is unlabelled

Where: frontend-app-course-authoring, Settings → Grading
(${APPS}/<mount>/course/<key>/settings/grading). Measured on Tutor main,
2026-09-08, on a fresh course (default Pass/Fail range).

Finding: axe reports two critical WCAG 2.2 AA violations in the editor's own
controls, independent of authored content:

• button-name (1 node per draggable boundary): the range handles
  (button.grading-scale-segment-btn-resize[role="slider"]) carry
  aria-valuemin/max/now but no accessible name, so a screen reader announces
  "slider, 50" with no indication of which grade boundary it moves.
• label (3 nodes on the default range, one per segment): the segment name
  fields (input[data-testid="grading-scale-segment-input"]) have no label,
  placeholder, or aria-label; the failing bucket's disabled field included.

Suggested fix: give each handle an aria-label naming the boundary it moves
(e.g. "Boundary between A and B"), and each segment field an aria-label
("Grade name") — the range text beside it is not associated with the input.

Start in frontend-app-course-authoring at Settings → Grading and locate the grading-scale resize handles and segment-name inputs using the selectors in the report. Rerun axe on a fresh course with the default Pass/Fail range; done means each boundary slider and segment field has an accessible name and the reported button-name and label violations are gone.