Add computeProgress to core and Storybook to @learnkit-ai/react

[raj-khan]

Summary

• computeProgress(path, completedIds) — new function in @learnkit-ai/core that returns a LearningPathProgress with completedCount, totalCount, percentComplete, and updatedAt; filters out IDs that don't belong to the path
• LearningPathProgressSchema extended with completedCount, totalCount, percentComplete fields
• Storybook set up in packages/react with @storybook/react-vite; stories for all four components (LessonCard, LearningPath, LessonDetail, ProgressTracker) covering every status, theme, and headless render-prop mode — run with pnpm --filter @learnkit-ai/react storybook

Test plan

• pnpm test passes (39 core + react + cli tests)
• pnpm typecheck passes across all packages
• pnpm --filter @learnkit-ai/react storybook starts and shows all four components with all theme variants
• computeProgress with empty IDs returns completedCount: 0, percentComplete: 0
• computeProgress with all lesson IDs returns percentComplete: 100
• Fake IDs are filtered from completedLessonIds