fix(graph): resolve color fallback so SVG nodes don't render black

[Jose-Gael-Cruz-Lopez]

Summary

• SVG presentation attributes (fill=) don't resolve CSS custom properties — when the backend doesn't supply a per-course color, the var(--c-sage) fallback was rendering black.
• Adds a deterministic hex palette + paletteFor(seed) helper in lib/data.ts and wires it into the Dashboard, Tree, and KnowledgeGraph fallback paths.
• Bumps subject-root node radius from 22 → 28 so course family centers read distinctly from concept nodes.

Test plan

• npx tsc --noEmit in frontend/ — clean
• Manual: load Dashboard / Tree with a course that has no course_color and confirm nodes render with a stable hue (not black)
• Manual: confirm subject-root nodes look noticeably larger than concept nodes
• Manual: same course id always maps to the same palette entry across screens

Note

A sibling fallback exists in screens/Learn.tsx#apiToGraphNode. It's intentionally left untouched here because PR #93 is in flight on that file. Once both land, a one-liner can switch Learn's fallback to paletteFor(...) for consistency.

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⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

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Run ID: 043bb35c-b363-4768-a23e-25a719776d3d

📥 Commits

Reviewing files that changed from the base of the PR and between 3fb23d3 and 84bb0e4.

📒 Files selected for processing (4)

• frontend/src/components/KnowledgeGraph.tsx
• frontend/src/components/screens/Dashboard.tsx
• frontend/src/components/screens/Tree.tsx
• frontend/src/lib/data.ts

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/graph-color-fallback

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Status	Name	Latest Commit	Preview URL	Updated (UTC)
✅ Deployment successful! View logs	frontend	84bb0e4	Commit Preview URL Branch Preview URL	May 05 2026, 10:19 PM

[Jose-Gael-Cruz-Lopez]

Folded into #93 — keeping rename feature and graph color fix in a single PR.