feat: add Storybook stories for new UI components

[Gitsack]

Summary

• Added Storybook stories for 5 components: ErrorBoundary, DeleteMediaDialog, ConfirmDialog, AddMediaDialog, and DownloadProgressCard
• Covers multiple variants per component (error states, loading states, different media types, import vs download progress)
• Follows existing story conventions (autodocs, actions, decorators)

Test plan

• Run npm run storybook and verify all new stories render correctly
• Check ErrorBoundary default/fullPage/customFallback/noError stories
• Check DeleteMediaDialog remove/deleteFile modes across media types
• Check ConfirmDialog default/loading states
• Check AddMediaDialog movie/album/tvshow/author variants and loading/adding states
• Check DownloadProgressCard single/multiple/importing/empty states

🤖 Generated with Claude Code

[Gitsack]

Review: LGTM — Clean addition of Storybook stories for 5 components.

• All component files exist and are properly imported
• Stories follow existing conventions (autodocs, actions, decorators)
• Good variant coverage: error states, loading states, media types, progress states
• No test regressions (257/257 pass)
• Storybook rendering requires manual verification (cannot run Storybook in CI headlessly here)