feat(ST-313): merge sprint board into single three-panel view

- TriplePane component with two resizable dividers, localStorage persistence, mobile tabs
- SprintBoardClient replaces SprintBacklogClient + PlanningRightClient
- Left panel: Product Backlog (PBIs with stories to add to sprint)
- Middle panel: Sprint Backlog (stories in sprint, click to select, sortable)
- Right panel: TaskList for selected story
- /sprint/planning redirects to /sprint
- Remove PlanningLeft, PlanningRightClient, SprintBacklogClient

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

components/sprint/task-list.tsx

@@ -119,22 +119,28 @@ function EditSubmitButton() {
 }
 
 function CreateTaskForm({ storyId, sprintId, onDone }: { storyId: string; sprintId: string; onDone: () => void }) {
-  const [, formAction] = useActionState(
+  const [state, formAction] = useActionState(
     async (_prev: unknown, fd: FormData) => {
       const result = await createTaskAction(_prev, fd)
-      if (result?.success) onDone()
+      if (result?.success) { onDone(); return result }
+      if (result?.error) toast.error(typeof result.error === 'string' ? result.error : 'Aanmaken mislukt')
       return result
     },
     undefined
   )
   return (
-    <form action={formAction} className="flex gap-2 px-4 py-2 border-b border-border">
+    <form action={formAction} className="flex flex-col gap-1.5 px-4 py-2 border-b border-border">
       <input type="hidden" name="storyId" value={storyId} />
       <input type="hidden" name="sprintId" value={sprintId} />
       <input type="hidden" name="priority" value="2" />
-      <Input name="title" autoFocus placeholder="Taaknaam…" className="h-7 text-sm flex-1" required />
-      <CreateSubmitButton />
-      <Button type="button" variant="ghost" size="sm" className="h-7" onClick={onDone}>×</Button>
+      <div className="flex gap-2">
+        <Input name="title" autoFocus placeholder="Taaknaam…" className="h-7 text-sm flex-1" required />
+        <CreateSubmitButton />
+        <Button type="button" variant="ghost" size="sm" className="h-7" onClick={onDone}>×</Button>
+      </div>
+      {state && 'error' in state && typeof state.error === 'string' && (
+        <p className="text-xs text-destructive">{state.error}</p>
+      )}
     </form>
   )
 }
@@ -219,6 +225,7 @@ export function TaskList({ storyId, sprintId, productId: _productId, tasks, isDe
           </div>
         ) : (
           <DndContext
+            id="task-list"
             sensors={sensors}
             collisionDetection={closestCenter}
             onDragStart={e => setActiveDragId(e.active.id as string)}