Admin Dashboard Recreated from CSR Manager Dashboard

Changed files:
- admin/dashboard.php
- admin/ajax/get_admin_manager_dashboard.php

What changed:
1. Admin overview now uses the CSR Manager dashboard layout/style and admin sidebar.
2. Admin dashboard uses admin-wide data for all teams, not just one manager/team.
3. Kept the CSR Manager-style Overall Total and Today stat containers.
4. Integrated old/admin-needed dashboard charts:
   - Team Performance
   - Status Summary / Task Status
   - Top City Used by CX, shown with region when available
   - Top Brands
   - Top Appliance
   - Urgency Distribution
   - Peak Hours
5. Kept admin quick links: Requests, Staff, CSR Teams, Announcements, Mistake, Warning, Admin Reports, Activity Logs, CSR Ranking.
6. Number/icon colors are still applied to the dashboard stat values only; card/container background stays unchanged.

Validation:
- PHP syntax checked for admin/dashboard.php
- PHP syntax checked for admin/ajax/get_admin_manager_dashboard.php
- PHP syntax checked for all admin PHP files
