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>

lib/schemas/sprint.ts

@@ -0,0 +1,38 @@
+import { z } from 'zod'
+
+const dateField = z.string().optional().nullable().transform(v => (v && v.trim() !== '' ? new Date(v) : null))
+
+export function validateDateOrder(
+  data: { start_date: Date | null; end_date: Date | null },
+  ctx: z.RefinementCtx,
+) {
+  if (data.start_date && data.end_date && data.end_date < data.start_date) {
+    ctx.addIssue({ code: z.ZodIssueCode.custom, path: ['end_date'], message: 'Einddatum moet na startdatum liggen' })
+  }
+}
+
+export const createSprintSchema = z
+  .object({
+    productId: z.string(),
+    sprint_goal: z.string().min(1, 'Sprint Goal is verplicht').max(500),
+    start_date: dateField,
+    end_date: dateField,
+  })
+  .superRefine(validateDateOrder)
+
+export const updateSprintDatesSchema = z
+  .object({
+    id: z.string(),
+    start_date: dateField,
+    end_date: dateField,
+  })
+  .superRefine(validateDateOrder)
+
+export const updateSprintGoalSchema = z.object({
+  id: z.string(),
+  sprint_goal: z.string().min(1, 'Sprint Goal is verplicht').max(500),
+})
+
+export type CreateSprintInput = z.infer<typeof createSprintSchema>
+export type UpdateSprintDatesInput = z.infer<typeof updateSprintDatesSchema>
+export type UpdateSprintGoalInput = z.infer<typeof updateSprintGoalSchema>