feat(whiteboard): move 'Add brush' to menu

[BrayanDSO]

It is less discoverable, but the Add button isn't a frequent action, so it's better to avoid occupying screen space if the user doesn't have many brushes.

How Has This Been Tested?

Emulator 31

Screen_recording_20251230_052827.mp4

Checklist

Please, go through these checks before submitting the PR.

• You have a descriptive commit message with a short title (first line, max 50 chars).
• You have commented your code, particularly in hard-to-understand areas
• You have performed a self-review of your own code
• UI changes: include screenshots of all affected screens (in particular showing any new or changed strings)
• UI Changes: You have tested your change using the Google Accessibility Scanner

[lukstbit]

I'm ok with the change.
We should consider still showing the 'add' option(maybe the other menu options as well?) where screen size allows, ex. tablets.

[BrayanDSO]

We should consider still showing the 'add' option(maybe the other menu options as well?) where screen size allows, ex. tablets.

Sure. That's a great idea