Summary:
Replaced three separate _openCategory() implementations with a single, flexible showCategorySelectionDialog() function.

Details:
Previously, there were three distinct _openCategory() methods used in different contexts (bulk manga selection, single manga update, and library addition). All three shared nearly identical UI logic and structure, leading to duplicated code and maintenance complexity.

This PR introduces showCategorySelectionDialog(), a unified, parameterized function that:

• Supports both single manga and bulk manga inputs via assertion logic
• Conditionally shows/hides hidden categories (just like before, shows hidden categories, when adding to library)
• Replaces all redundant methods with a reusable, streamlined implementation

Benefits:

• Simplifies codebase by removing duplication
• Improves maintainability and readability
• Reduces future bugs due to logic divergence

Testing:

• Verified behavior with both single manga updates and bulk selection
• Ensured sync operations are preserved for both update and add scenarios
• Confirmed categories render and update as expected in all entry points