Add v1 polish: LessonDetail, light theme, renderItem, OG images, prerequisite graph

[raj-khan]

Summary

• <LessonDetail />: new React component — renders lesson body (2 paragraphs), exercises (prompt + expected output + reviewer note), and rubric (excellent / acceptable / needs work). Uses generateLessonContent() internally. Supports all 4 themes.
• light theme: 4th theme variant for LearningPath, ProgressTracker, and LessonDetail — indigo/pink/emerald palette on white.
• renderItem render-prop: added to both LearningPath and ProgressTracker. When provided, called per-lesson instead of <LessonCard> — enables fully custom lesson rendering without forking the components.
• Lesson prerequisite graph: prerequisiteIds: string[] added to LessonSchema. generateLearningPath wires sequential prerequisites (each lesson depends on the previous). ProgressTracker uses isEffectiveDone to lock lessons whose prerequisites aren't effectively complete — including cascade-locking on un-complete.
• OG images: dynamic ImageResponse for /roles/[slug] (warm paper background) and /tools/[slug] (dark brand background). Edge runtime.
• Demo LessonPreview: now renders real lesson content (title, body, first exercise, rubric hint) from generateLessonContent() rather than static hardcoded text. Lesson shown reflects the user's role, tools, goal, and level choices.
• DemoFlow: roles updated to include Sales, Customer Success, Finance; tools updated to include Windsurf, Replit, Linear, Figma AI, v0.

Test plan

• All typechecks pass: schemas, core, react, cli, web
• 33 core tests pass (3 new prerequisite tests)
• 39 react tests pass (7 new LessonDetail, 3 new LearningPath, 3 new ProgressTracker)
• CI green on all matrix nodes

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