UI styles rework

[adriengivry]

Description

• ImGui styles are now fully abstracted
• This new abstraction layer allows us to reference styles anywhere in the engine, and maintain consistent visuals
• Changing styles automatically update button colors, no need to restart the editor
• Light themes have been updated to look nicer
• No more EDITOR_UI_SCALE, we now use OVUI_SCALE

Related Issue(s)

Fixes #(issue number)

Review Guidance

Write here.

Screenshots/GIFs

AI Usage Disclosure

Claude Sonnet 4.6 did it all for me

Checklist

• My code follows the project's code style guidelines
• When applicable, I have commented my code, particularly in hard-to-understand areas
• When applicable, I have updated the documentation accordingly
• My changes don't generate new warnings or errors
• I have reviewed and take responsibility for all code in this PR (including any AI-assisted contributions)

[Gopmyc]

LGTM ! (I haven't tested it)