Add user-select CSS for line numbers in file viewer

[stescobedo92]

Add CSS to disable text selection for line numbers

Fixes #4856

Description

This pull request improves the user experience when copying code from the file viewer and diff views. Currently, selecting text often includes the line numbers, which interferes with copying code snippets.
I have added user-select: none (and its vendor-prefixed variants) to the CSS classes responsible for rendering line numbers (.l, .hl, del.d, etc.). This ensures that line numbers are skipped during text selection while ensuring links and other interactions remain functional.

Changes

• Modified [opengrok-web/src/main/webapp/default/style-1.0.4.css] to apply selection disabling styles to line number classes.

Verification

• Validated that the modified CSS classes (.l, .hl, del.d) are correctly used for line numbers in [diff.jsp] and other views.
• Verified that user-select is the standard approach for this behavior and does not affect the ability to click on line number links (if applicable).

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Pull request overview

This pull request enhances the user experience when copying code from OpenGrok's file viewer by preventing line numbers from being included in text selections. The change applies the user-select: none CSS property (with vendor prefixes for broader browser compatibility) to the CSS classes responsible for rendering line numbers across different views.

Changes:

• Added user-select: none CSS property to line number elements in the stylesheet

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