Improve placeholder and side menu icon contrast for WCAG compliance

[YousefED]

Discussed in #2519

^{Originally posted by Ovgodd September 4, 2025}
Is your feature request related to a problem? Please describe.
The placeholder text and side menu icons in the BlockNote editor have insufficient contrast.
This fails WCAG 2.1 requirements (minimum 4.5:1), making it harder for users with low vision.

Describe the solution you'd like
Update the placeholder text and side menu icon styles to use higher-contrast colors.
For example, switching --bn-colors-side-menu: #cfcfcf; to --bn-colors-side-menu: #2a2a2a;