[FE] Manage & Apply Competencies: delete a Competency Criteria Association

Use Case

As a course author who associated the wrong gradeable subsection with a Competency Criteria Group, or no longer wants a subsection counted toward a competency, I want to remove a single association from a Competency Criteria Group box's Selected Content list, so that I can correct the rule's content without deleting or recreating the whole box.

Acceptance Criteria

Frontend ticket. Testable once #672's Selected Content list and #670's Browse Courses panel chip exist, #674 exists, and #709's group-removal rendering logic exists.

Scenario: Remove an association via its Competency Criteria Group box item
  Given a gradeable subsection is associated with a Competency Criteria Group box, among other associations in the same box
  When I click the remove ("X") affordance on that association's item in the box's Selected Content list
  Then that association is removed
  And the item no longer appears in the box's Selected Content list
  And the subsection's chip in the Browse Courses panel no longer shows it as already-associated, and it is selectable again
  And the box's other associations remain unchanged

Scenario: Remove an association via its Browse Courses panel chip
  Given a gradeable subsection is associated with a Competency Criteria Group box, among other associations in the same box
  When I click the remove ("X") affordance on that subsection's already-associated chip in the Browse Courses panel
  Then that association is removed
  And the subsection's chip in the Browse Courses panel no longer shows it as already-associated, and it is selectable again
  And the item no longer appears in the box's Selected Content list
  And the box's other associations remain unchanged

Scenario: Removing the last association empties a box with siblings present
  Given a Competency Criteria Group box has exactly one association, and other boxes exist for the competency
  When I remove that association, from either its Selected Content item or its Browse Courses panel chip
  Then the now-empty box is removed from the page
  And the competency's other boxes remain unaffected

Scenario: Removing the last association from the only remaining box
  Given a Competency Criteria Group box has exactly one association, and it is the only box for the competency
  When I remove that association, from either its Selected Content item or its Browse Courses panel chip
  Then the box is removed
  And a placeholder box is shown as the new implicit active target

Scenario: Backend rejects the removal
  Given a remove request to the backend fails
  When I click the remove affordance on an association, from either entry point
  Then the association remains visible in the box's Selected Content list, and its Browse Courses panel chip still shows it as associated
  And an error is shown, following this feature's existing generic-error-handling pattern

Scenario: A course author without write access cannot remove an association
  Given I lack studio write access to the course the association's subsection belongs to
  Then neither the Selected Content item's remove affordance nor the Browse Courses panel chip's remove affordance is rendered

Screenshot below illustrates some of the workflow, but the Figma tells the story.

Description

This ticket adds a remove/deselect action to a single Selected Content item, letting a course author remove one subsection's association from a Competency Criteria Group box without affecting that box's other associations.

Current behavior

An associated subsection is currently visible in two places at once: as an item in its Competency Criteria Group box's Selected Content list (SelectedContentItem, built "display only, no deselect action" by #672), and as an already-associated chip on the subsection itself in #670's Browse Courses panel. Neither currently offers a way to remove the association; the only "undo" available is deleting the entire Competency Criteria Group box (5.10c), which removes every other association in it too.

Requested change

• Both of that association's existing chips gain a remove/deselect "X": the one on its item in the Competency Criteria Group box's Selected Content list, and the one on the subsection's own chip in #670's Browse Courses panel.
• Clicking either X removes the same one association via #674; the Competency Criteria Group box, its logic operator, and its other associations are unaffected. Both surfaces are rendered from the same GET-backed association data, so removing from either X updates both at once: the item disappears from the box's Selected Content list, and the subsection's chip in the Browse Courses panel visibly reverts to unassociated, selectable again.
• If removing an association empties its Competency Criteria Group box, #674 deletes that now-empty box in the same call (see Technical Details). The box simply disappears from the page if sibling boxes remain, or the panel falls back to the placeholder box if it was the last one, per 5.10c's existing rendering logic, reused here rather than re-implemented.