feat(ST-cmovhvej7): add PBI_HAS_ACTIVE_TASKS alongside-dialoog in materialize handler

When materializeIdeaPlanAction returns code 409 with PBI_HAS_ACTIVE_TASKS:<code>,
a confirm dialog offers the user a choice: create new PBI alongside the existing one
or cancel. Alongside=true retries the action; cancel leaves the idea in PLAN_READY.

components/ideas/idea-row-actions.tsx

@@ -107,15 +107,24 @@ export function IdeaRowActions({ idea, isDemo, onArchive }: IdeaRowActionsProps)
   function handleMaterialize() {
     if (!confirm('Plan materialiseren? Dit maakt PBI + stories + taken aan.')) return
     startTransition(async () => {
-      const r = await materializeIdeaPlanAction(idea.id)
+      let r = await materializeIdeaPlanAction(idea.id)
+
+      if ('error' in r && r.code === 409 && r.error.startsWith('PBI_HAS_ACTIVE_TASKS:')) {
+        const pbiCode = r.error.split(':')[1]
+        const alongside = confirm(
+          `De bestaande PBI (${pbiCode}) heeft uitgevoerde taken.\n` +
+          `OK = nieuwe PBI naast bestaande aanmaken.\n` +
+          `Annuleren = stoppen.`
+        )
+        if (!alongside) return
+        r = await materializeIdeaPlanAction(idea.id, { allowAlongside: true })
+      }
+
       if ('error' in r) {
         toast.error(r.error)
         return
       }
       toast.success(`Gematerialiseerd als ${r.data?.pbi_code}`)
-      // Navigeer naar de product-backlog. Anchor-scrolling per-PBI bestaat
-      // (nog) niet in pbi-list, dus gewoon naar de overview-pagina; de nieuwe
-      // PBI is de meest recente.
       if (idea.product_id) {
         router.push(`/products/${idea.product_id}`)
       } else {