feat(ST-1205): add sprint start_date/end_date UI + server actions

- createSprintAction + updateSprintDatesAction: Zod date validation
  with end_date >= start_date cross-check
- start-sprint-button: date inputs in create dialog
- sprint-header: date display button + edit dialog with updateSprintDatesAction
- sprint page: select start_date/end_date for SprintHeader prop
- Demo blokkade via bestaande isDemo checks
- 6 tests groen (validation + demo guard)

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

app/(app)/products/[id]/sprint/page.tsx

@@ -33,6 +33,13 @@ export default async function SprintBoardPage({ params, searchParams }: Props) {
 
   const sprint = await prisma.sprint.findFirst({
     where: { product_id: id, status: 'ACTIVE' },
+    select: {
+      id: true,
+      sprint_goal: true,
+      status: true,
+      start_date: true,
+      end_date: true,
+    },
   })
   if (!sprint) redirect(`/products/${id}`)