feat(ST-607): WCAG AA accessibility fixes

- PBI row: role="button", tabIndex, aria-selected, onKeyDown (Enter/Space), focus-visible ring
- PBI "+" button: aria-label with priority name
- Sprint header cancel button: aria-label="Annuleer bewerken"
- Task create cancel button: aria-label="Annuleer"

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

components/sprint/sprint-header.tsx

@@ -84,7 +84,7 @@ export function SprintHeader({ productId: _productId, productName, sprint, isDem
               <Textarea name="sprint_goal" defaultValue={sprint.sprint_goal} rows={2} className="text-sm flex-1" autoFocus />
               <div className="flex flex-col gap-1">
                 <SaveGoalButton />
-                <Button type="button" size="sm" variant="ghost" onClick={() => setEditingGoal(false)}>×</Button>
+                <Button type="button" size="sm" variant="ghost" aria-label="Annuleer bewerken" onClick={() => setEditingGoal(false)}>×</Button>
               </div>
             </form>
           ) : (