feat(ST-108/ST-208): replace inline forms with PBI and story dialogs

- PbiDialog: create/edit with priority select and optional description
- StoryDialog: create/edit with priority, description, acceptance criteria, activity log, and delete
- PrioritySelect: reusable controlled select component
- Edit icons always visible on PBI rows and story blocks
- Dialog backdrop uses 40% opacity blur

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

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

@@ -89,7 +89,7 @@ export default async function ProductBacklogPage({ params }: Props) {
           left={
             <PbiList
               productId={id}
-              pbis={pbis.map((p: (typeof pbis)[number]) => ({ id: p.id, title: p.title, priority: p.priority }))}
+              pbis={pbis.map((p: (typeof pbis)[number]) => ({ id: p.id, title: p.title, priority: p.priority, description: p.description }))}
               isDemo={isDemo}
             />
           }