Feat/video call join links

[Alaa2019-ml]

In this pull request:

• Added a Start Call button in the TeacherAppointments appointment row to initiate calls.
• Added a confirmation modal before starting a call.
• Implemented late-join support: expired calls are marked as missed, and students can still join within 3 hours.
• Extended the call expiry when call is accepted to allow students to join and rejoin the call.
• Added join link attachment to appointments for valid calls (ringing, accepted, recent missed), allowing students to join or rejoin directly from the appointments page.
• Added configurable modal props: confirmText, cancelText, and confirmVariant for flexible button labels and styling.