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>

components/ui/dialog.tsx

@@ -31,7 +31,7 @@ function DialogOverlay({
     <DialogPrimitive.Backdrop
       data-slot="dialog-overlay"
       className={cn(
-        "fixed inset-0 isolate z-50 bg-black/10 duration-100 supports-backdrop-filter:backdrop-blur-xs data-open:animate-in data-open:fade-in-0 data-closed:animate-out data-closed:fade-out-0",
+        "fixed inset-0 isolate z-50 bg-black/40 backdrop-blur-sm duration-100 data-open:animate-in data-open:fade-in-0 data-closed:animate-out data-closed:fade-out-0",
         className
       )}
       {...props}