Resolve "✨Feat: Predefined Specialization Dropdown in Admin “Add Doctor” Popup"

Closes #135 (closed)

Summary

This MR replaces the free-text Specialization field in the Admin Dashboard’s Add Doctor popup with a predefined dropdown list of specializations.

Why

Manual text entry was causing inconsistent specialization names, typos, and duplicate variants, which impacts filtering, reporting, and future specialization-based workflows.

What Changed

• Replaced specialization text input with a required dropdown/select.
• Added a predefined specialization list (single source of truth).
• Updated form validation to accept only supported specialization values.
• Ensured selected specialization is sent in create-doctor payload and rendered correctly in doctor cards/listing.

Impact

• Standardized specialization data across the app.
• Reduced data-entry errors during doctor onboarding.
• Improved readiness for specialization-based queueing/assignment features.

Testing Done

• Verified Add Doctor modal renders specialization as dropdown.
• Verified form blocks submission when specialization is not selected.
• Verified successful doctor creation with selected specialization.
• Verified specialization appears correctly in Doctors Directory UI.
• Verified no regression in other Add Doctor form fields and submission flow.