feat: font size adjustment slider with pip/notch indicators in Interface Settings

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Is your feature request related to a problem? Please describe.
The current interface settings lack a user-friendly method for adjusting font sizes, forcing users to rely on either manual editing of CSS files and rebuilding the front-end or resorting to the use of browser zoom features, which can be inconvenient and affect the overall layout of the interface.

Describe the solution you'd like
A font size range slider with pip or notch indicators along the font size range should be added to the Interface settings of Open WebUI. This would provide a straightforward and intuitive way to adjust font sizes in real-time, with clear visual indicators of available font sizes, enhancing the overall customization experience.

Describe alternatives you've considered
Alternative solutions, such as third-party browser extensions or direct CSS editing, can be limited in their compatibility and functionality, and may require technical expertise. A native solution within Open WebUI would provide a more seamless and accessible experience for users.

Additional context
Implementing a font size range slider with pip or notch indicators along the font size range would enhance the usability and accessibility of Open WebUI, catering to diverse user needs and preferences. This feature would be particularly beneficial for users with visual impairments or those who require specific font sizes for optimal readability.