Add Exam Dates management section in Admin Dashboard

[DreamBot706]

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.