Implement leave request

[maurya-doshi]

Implement leave request

Summary

Adds a complete leave request workflow for peer evaluations, allowing students to request leave from exams and teachers to approve/reject those requests. Also includes an incentivization system where students can volunteer to grade extra papers from a pool.

Features

Student Side

• Students can submit leave requests for specific exams with a reason.
• Students can view their leave request history and current status (pending/approved/rejected).
• Students can claim extra evaluation papers from an incentivized pool.
• Approved leave requests automatically hide the associated peer evaluations from the student's Peer Evaluation tab.

Teacher Side

• Teachers can view all pending leave requests filtered by exam.
• Teachers can approve or reject leave requests.
• Approving a leave creates extra pool evaluations that other students can claim for incentive points.

Peer Evaluation Integration

• initiatePeerEvaluation now excludes students on approved leave from the active evaluator pool.
• Extra pool evaluations (with isExtra: true) are created for students on leave so their papers still get graded.
• getPendingEvaluations filters out evaluations for exams where the student has approved leave.

New Files

• backend/src/models/LeaveRequest.ts — Mongoose model for leave requests.
• backend/src/controllers/student/leave.controller.ts — Student leave endpoints.
• backend/src/controllers/teacher/leave.controller.ts — Teacher leave review endpoint.
• frontend/src/components/student/StudentLeaves.tsx — Student leave UI.
• frontend/src/components/teacher/TeacherLeavesRequests.tsx — Teacher leave management UI.

Bug Fixes

• Fixed exam dropdowns showing blank/undefined by using exam.title instead of exam.name across frontend components.
• Fixed duplicate navigation block in Login.tsx.
• Removed debug console.log statements in OtpScreen.tsx.

Cleanup

• Removed dead commented-out imports and routes in server.ts.
• Fixed .gitignore — removed overly broad **/*.json rule that was suppressing package.json/tsconfig.json, added proper ignores for .env, .zip, temp files, and log files.