refactor: DRY and dead-code cleanup in StoryViewer
- Remove nextStoryFromTrailing state var and its .transition(.asymmetric)
on the story image — within-story nav is never wrapped in withAnimation
so the transition never fired
- Consolidate showReportSheet + reportStoryUri + reportStoryCid (3 vars)
into reportTarget: GrainStory? using fullScreenCover(item:)
- Extract advanceStory(by:) to eliminate duplicated 5-line state reset
in goToNext and goToPrevious