feat(backlog): migrate StoryPanel to store-driven + selectStory on click

Removes storiesByPbi prop; reads from useBacklogStore. Card click now
dispatches selectStory(id) + shows isSelected highlight. Edit moved to
inline pencil button. page.tsx drops pbis/storiesByPbi props.

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

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

@@ -137,13 +137,11 @@ export default async function ProductBacklogPage({ params, searchParams }: Props
               <PbiList
                 key="pbi"
                 productId={id}
-                pbis={pbis.map((p: (typeof pbis)[number]) => ({ id: p.id, code: p.code, title: p.title, priority: p.priority, description: p.description, created_at: p.created_at, status: pbiStatusToApi(p.status) }))}
                 isDemo={isDemo}
               />,
               <StoryPanel
                 key="story"
                 productId={id}
-                storiesByPbi={storiesByPbi}
                 isDemo={isDemo}
               />,
               <TaskPanel