fix: black background in story viewer light mode and clean warnings
Story viewer was showing white safe-area bars in light mode because the
black background lived inside a clipped ZStack. Move it outside the clip
so it extends through the safe areas. Also silence two compiler
warnings: an unused `story` binding in the delete button and main-actor
isolation on the tab bar badge appearance helper.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>