feat(ui): add keyboard navigation and focus states for code blocks

[Tugamer89]

What:
Added tabindex="0" to all <pre> blocks via Javascript injected through pom.xml during Javadoc generation.
Added :focus-visible CSS styles for <pre> elements in custom-javadoc.css.

Why:
Code blocks can overflow and require horizontal scrolling. Without tabindex="0", users navigating primarily by keyboard cannot focus the blocks to scroll them, making the content inaccessible.

Accessibility:
Improves keyboard accessibility for reading code snippets by providing an interactive focus target with a visible focus ring (matching existing focus tokens in the project) for horizontal scrolling.

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PR created automatically by Jules for task 17938638575101798008 started by @Tugamer89

[google-labs-jules]

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud