Add grayscale to palette, add as button color

[gigxz]

Description

Issue: https://github.com/open-path/Green-River/issues/7130
Chromatic: Link

This PR is a refactor of "gray" buttons in the app, to match the hmis design library.

• Add grayscale color palette per design library
• Replace usage of old gray button variant color color=grayscale variant=contained
• Replace usage of old transparent button variant with color=grayscale variant=text
• Update TODOs
• Add button variant/color stories
• Remove default white background from outlined buttons per figma change (see https://github.com/open-path/Green-River/issues/7130#issuecomment-2560383390). This allows them to be used on alerts correctly.

Type of change

• Code clean-up / housekeeping

Checklist before requesting review

• I have performed a self-review of my code
• I have run the code that is being changed under ideal conditions, and it doesn't fail
• My code includes comments and/or descriptive variable names to help other engineers understand the intent (or not applicable)
• My code follows the style guidelines of this project (eslint)
• I have updated the documentation (or not applicable)
• If it's not obvious how to test this change, I have provided testing instructions in this PR or the related issue