feat: #1600 Admin Search Across All Data

[anshika1179]

What does this PR do?

Resolves #1600. Adds global search functionality to the Admin Dashboard. Implements /api/admin/search endpoint that queries Users, Events, and Announcements via Prisma. Also integrates a sidebar search button and wires the backend to the CommandMenu modal, fulfilling the <200ms and Search across all data requirements.

Fixes #1600

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Code refactoring / optimization

How Has This Been Tested?

• Tested locally
• Verified UI/UX responsiveness
• Checked for console warnings and errors

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• I have checked my code and corrected any misspellings

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