Add navbar icons

[AJaccP]

🧠 Context

src/components/Header.tsx renders the top navigation bar with two <NavLink> items: Explorer and Planner. They're currently text-only. Adding icons from lucide-react makes the nav more visually distinct.

---

🛠️ Implementation Plan

1. Open src/components/Header.tsx and import appropriate icons. Suggested icons:

   • Explorer: Network (represents a graph/node structure)
   • Planner: CalendarDays (represents term-by-term planning)

   Feel free to browse lucide.dev if you'd prefer different icons, just keep them intuitive.

2. Render each icon inline with its label inside the <NavLink>, aligned horizontally with a small gap between the icon and text.

3. Run pnpm dev and verify both links show their icons, the active-state colour (text-red-600) applies to both icon and text, and the layout looks correct.

---

✅ Acceptance Criteria

• Explorer nav link has an icon to its left
• Planner nav link has an icon to its left
• Active-state colour applies to both the icon and the text
• Icons are visually consistent in size and alignment
• pnpm typecheck and pnpm test still pass

[PulkitSharma762]

Hi, I would like to work on this issue.