Color-code course nodes by year

[AJaccP]

As a student using the Explorer, I want course nodes to be color-coded by year level so I can quickly see the progression of the curriculum.

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🧠 Context

src/components/CourseNode.tsx renders each node in the prerequisite graph. All nodes currently have the same border-gray-300 border regardless of course level.

Course codes follow the pattern DEPT XXXX (e.g. COMP 1405, COMP 3004) — the first digit of the numeric part indicates the year level: 1xxx, 2xxx, 3xxx, 4xxx.

The change is purely visual and isolated to CourseNode.tsx — no store changes, no new components.

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🛠️ Implementation Plan

1. Open src/components/CourseNode.tsx.

2. Derive the year level from data.code by extracting the first digit of the numeric portion of the course code.

3. Replace the static border-gray-300 with a border color that varies by year level. Pick four visually distinct colors — one per level. Keep them readable against the white node background.

4. Run pnpm dev, open the Explorer, and verify nodes at each year level display a distinct border color.

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✅ Acceptance Criteria

• 1xxx, 2xxx, 3xxx and 4xxx courses have distinct border colors
• All four colors are visually distinct from each other and readable against the white background
• Existing node behaviour (click to highlight, dim on selection) is unchanged
• pnpm typecheck and pnpm test still pass