Editor: Improve text/background color discoverability in toolbar

[im3dabasia]

What?

Closes #64539

Rename the "Highlight" toolbar button to "Text color" and update its icon to the standard A icon with a color bar, matching Canva and Google Docs conventions.

Why?

"Highlight" is misleading, users can't find text color controls.

How?

• Rename label: Highlight → Text color
• Update icon to A with a color indicator bar
• Optionally split into separate Text color and Highlight color items

Testing Instructions

1. Add some text.
2. Select the text and open the toolbar.
3. Click More, then add a text color and a background color.
4. Apply the changes and check the frontend.

Screenshots or screencast

[github-actions]

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