This PR introduces three distinct improvements and bug fixes to enhance the user experience and app stability:

1. Change Button Behavior of "Edit Categories" Inside a Library Item's Detail View

(manga_detail_view.dart)

• Renamed the "Edit Categories" button to "Set Categories".
• The button now opens the category select menu instead of the category edit screen.
• This change makes it easier for users to quickly update an item's category (e.g., from "Watching" to "Finished") directly from the detail view.
• Previously, users had to exit the detail view and take multiple steps in the library view to change the category.
   

2. Add Switch to Automatically Check for Updates in Settings > About

(settings.dart; about_screen.dart; check_for_update.dart)

• Introduced a toggle in the app settings allowing users to enable or disable automatic update checks.
• Default behavior changed from always checking to letting the user choose.
• Added corresponding translations for this new setting.
   

3. Fix Exception: _AssertionError in NavigationRail

(main_screen.dart)

• Resolved a crash caused by an out-of-bounds selectedIndex when customizing the navigation and hiding certain items.
• Ensured the NavigationRail component now verifies that selectedIndex is within a valid range
• Fixes the following error:

Exception has occurred.
_AssertionError ('package:flutter/src/material/navigation_rail.dart': Failed assertion: line 112 pos 15: 'selectedIndex == null || (0 <= selectedIndex && selectedIndex < destinations.length)': is not true.)

4. Fixes #478

• Fully use join function instead of hacks like fixSeparator, which lead to issues as seen here.