Add Exam Dates management section in Admin Dashboard

## Description
Currently, setting exam dates for each semester requires manual database entry. We need a dedicated section in the Admin Dashboard to easily configure and update these dates.

## Schema Context
Uses the existing `Event` model (`server/modules/event/event.model.js`):
- `firstYearDates`: `{ midSem: Date, endSem: Date }`
- `otherDates`: `{ midSem: Date, endSem: Date }`

## Requirements
- Add an **Exam Dates** page/section in the Admin Dashboard with a link in the sidebar.
- Provide UI inputs to view and set Mid-Sem and End-Sem dates for both **1st Year** and **2nd+ Year** students.
- Add an admin-authenticated API endpoint to update/upsert these dates.
- Display toast notifications on save/error.