feat: voeg geselecteerde PBI automatisch toe aan nieuwe sprint

Bij sprint-aanmaak wordt de pbi_id uit de selection-store als hidden
form-field meegestuurd. Server-side worden alle stories van die PBI
(zonder sprint) en hun taken aan de nieuwe sprint gekoppeld; stories
krijgen status IN_SPRINT met incrementele sort_order.

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

components/sprint/start-sprint-button.tsx

@@ -21,6 +21,7 @@ import {
   entityDialogHeaderClasses,
 } from '@/components/shared/entity-dialog-layout'
 import { createSprintAction } from '@/actions/sprints'
+import { useSelectionStore } from '@/stores/selection-store'
 
 interface StartSprintButtonProps {
   productId: string
@@ -44,6 +45,7 @@ export function StartSprintButton({ productId, isDemo = false }: StartSprintButt
   const [dirty, setDirty] = useState(false)
   const formRef = useRef<HTMLFormElement>(null)
   const router = useRouter()
+  const selectedPbiId = useSelectionStore((s) => s.selectedPbiId)
 
   const [state, formAction, pending] = useActionState<ActionResult | undefined, FormData>(
     async (_prev, fd) => {
@@ -92,6 +94,7 @@ export function StartSprintButton({ productId, isDemo = false }: StartSprintButt
             className="flex-1 overflow-y-auto px-6 py-6 space-y-6"
           >
             <input type="hidden" name="productId" value={productId} />
+            {selectedPbiId && <input type="hidden" name="pbi_id" value={selectedPbiId} />}
 
             <div className="space-y-1.5">
               <label className="text-sm font-medium text-foreground">