Add Select All / Unselect All toggle functionality - Fixes #1566

[Tejus468]

This PR implements a "Select All" / "Unselect All" toggle button for the photo management interface as requested in issue #1566.

Changes Made

• Added a "Select All" button to the templates/manage-photos.php template
• Implemented JavaScript functionality to toggle between selecting all photos and unselecting all photos
• The button text dynamically changes between "Select All" and "Unselect All" based on the current state
• Integrated with the existing OP.Batch system for consistent photo selection behavior

Features

• Select All: Clicking when no photos are selected will select all visible photos
• Unselect All: Clicking when photos are selected will clear all selections
• Dynamic Button Text: Button text updates automatically based on selection state
• Seamless Integration: Works with existing individual photo selection functionality

Testing

• Tested the toggle functionality with mock data
• Verified button state changes correctly
• Confirmed integration with existing batch selection system

Fixes

Closes #1566

Screenshots

The Select All button is positioned between the pagination and photo gallery for easy access.