Add labels style command to change Gmail label colors #456
Description
Problem
gog gmail labels supports list, get, create, rename, modify, and delete, but there's no way to change label colors from the CLI.
The Gmail API supports setting label colors via labels.patch with the color field.
Proposed usage
gog gmail labels style --background-color "#16a765" --text-color "#ffffff"
Both --background-color and --text-color are required (Gmail API requires both). Colors must be from the predefined palette. System labels are blocked, consistent with rename/delete.