The rich-text editor already registers the TextStyle and Color
extensions so that colored text pasted or quoted from incoming mail
survives editing - but there was no way to set a color yourself.
Adds a "Text color" toolbar button next to the strikethrough control,
wired to the already-loaded extensions: a 2x8 preset swatch grid plus a
"Remove color" entry, following the table button's dropdown pattern
(wrapper ref, outside-click close, same popover styling) and the table
size picker's swatch grid. The button's baseline icon renders in the
currently active color, so the selection is visible without any extra
indicator element.
No new dependencies and no locale changes; the toolbar titles in this
file are plain English throughout, and Clear Formatting already removes
colors via unsetAllMarks.
