Add full course title on hover

[AJaccP]

As a student using the Explorer, I want to see the full course title when hovering over a node so I don't have to remember what each code means.

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🧠 Context

src/components/CourseNode.tsx renders each node in the prerequisite graph. The node shows the course code (e.g. COMP 2402) and a truncated version of the title below it — both with Tailwind's truncate class, so long titles are cut off at the node boundary.

On desktop, adding a native title attribute to the root element is enough to show the full title on hover via the browser tooltip. This won't work on mobile (no hover), but that's acceptable for now — a proper course detail panel is a future ticket.

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🛠️ Implementation Plan

1. Open src/components/CourseNode.tsx.
2. Add title={data.title} to the outermost <div> of the returned JSX. That's the div with className="flex flex-col justify-center rounded border...".
3. Run pnpm dev, open the Explorer, and hover over a node with a long title (e.g. COMP 2402 — Abstract Data Types and Algorithms) to confirm the full title appears in the browser tooltip.

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✅ Acceptance Criteria

• Hovering a graph node on desktop shows the full course title in a browser tooltip
• No visual changes to the node itself
• Clicking a node still triggers the existing highlight behaviour
• pnpm typecheck and pnpm test still pass