Implement mentor schedule and availability planning system

[jjalangtry]

Why

The branch adds an advanced mentoring scheduling system with availability management and printable output, which is significantly beyond prior basic mentoring tasks.

Implemented scope

• Added schedule editor with drag/drop and slot management behavior.
• Added availability grid and printable schedule surfaces.
• Added schedule, schedule block, availability, and when2meet API routes.

Branches with implementation

• mentoring

Key files touched

• next/app/(main)/dashboard/mentoring/components/MentorScheduleEditor.tsx
• next/app/(main)/dashboard/mentoring/components/AvailabilityGrid.tsx
• next/app/(main)/dashboard/mentoring/components/PrintableSchedule.tsx
• next/app/api/mentorSchedule/route.ts
• next/app/api/scheduleBlock/route.ts
• next/app/api/mentor-availability/route.ts
• next/app/api/when2meet/route.ts

Follow-up checklist

• Add conflict detection and data-integrity validation tests.
• Validate timezone handling and daylight-savings edge cases.
• Add performance guardrails for larger schedules.