Add AI module installation hint when AI features unavailable

[jjroelofs]

Linked issues

• Show AI module installation hint when AI features unavailable #750

Solution

When the AI module is not installed, display a helpful hint in the Colors and Fonts theme settings sections. The message is styled with muted text and italics to be unobtrusive while still guiding users toward the AI features.

• Colors section: "Install the AI module to generate color palettes with AI."
• Fonts section: "Install the AI module to generate font pairings with AI."

Checklist

• I have read the CONTRIBUTING.md document.
• My commit messages follow the contributing standards and style of this project.
• My code follows the coding standards and style of this project.
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Need to run update.php after code changes
• Requires a change to end-user documentation.
• Requires a change to developer documentation.
• Requires a change to QA tests.
• Requires a new QA test.
• I have updated the documentation accordingly.
• All new and existing tests passed.