[feature]: ability to add and use custom fonts

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Summary

This is a feature request to introduce the capability to customize the UI font in self-hosted Plane instances.

Why should this be worked on?

The ability to set a custom font is essential for two primary reasons:

1. Enhanced Readability for International Teams: For users of languages with non-Latin scripts, such as Persian, the default font may not provide optimal legibility. Allowing a custom font would significantly improve the user experience and accessibility for these global users.

2. Brand Consistency: For businesses and organizations, using a specific brand font across all tools is crucial for maintaining a consistent and professional identity. The current limitation prevents this alignment.

By enabling font customization, Plane would become a more flexible and powerful tool for brand-conscious organizations and diverse international teams.