feat(sprint-dialogs): conform aan dialog-pattern + entity-profile

Story 5 van PBI "Alle dialogen conform docs/patterns/dialog.md".

- lib/schemas/sprint.ts — gedeelde zod-schemas (create/dates/goal)
- actions/sprints.ts — code+fieldErrors voor 422; code: 403 voor
  auth/demo errors
- StartSprintButton dialog: useDirtyCloseGuard, useDialogSubmitShortcut,
  entityDialog* layout-classes; DemoTooltip op trigger; veld-niveau
  errors via fieldErrors
- SprintHeader's date- en complete-dialogen: zelfde behandeling; date-
  dialog krijgt dirty-guard, complete-dialog krijgt DemoTooltip op
  bevestigen
- docs/specs/dialogs/sprint.md — entity-profile dat alle drie de modes
  documenteert; consolidatie naar één SprintDialog component bewust
  uitgesteld
- Sprint-dates tests aangepast aan nieuwe action-shape

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/INDEX.md

@@ -25,6 +25,7 @@ Auto-generated on 2026-05-04 from front-matter and headings.
 |---|---|---|
 | [PbiDialog Profiel](./specs/dialogs/pbi.md) | active | 2026-05-04 |
 | [ProductDialog Profiel](./specs/dialogs/product.md) | active | 2026-05-04 |
+| [Sprint Dialogs Profiel](./specs/dialogs/sprint.md) | active | 2026-05-04 |
 | [StoryDialog Profiel](./specs/dialogs/story.md) | active | 2026-05-04 |
 | [TaskDialog Profiel](./specs/dialogs/task.md) | active | 2026-05-03 |
 | [Scrum4Me — Functionele Specificatie](./specs/functional.md) | active | 2026-05-03 |