feat(PBI-49): add BEM sub-element data-debug-id to components/sprint/*
- new-sprint-dialog: __submit on submit button
- sprint-backlog: __list on SprintBacklogLeft + SprintBacklogRight scroll areas
- sprint-board-client: root wrapper div (display:contents) + __drag-overlay
- sprint-header: __title on goal button, __dates on dates button, __actions on action cluster
- sprint-run-controls: root on controls div, __start/__cancel on action buttons; __blockers-dialog on dialog content
- start-sprint-button: root on trigger button, __dialog on dialog content, __submit on submit button
- sync-active-sprint-cookie: no debug-id (returns null, side-effect only), comment added
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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