fix(chatgpt): correct user message text color

[JosephDemarest]

🗒 Checklist 🗒

• I have read and followed Catppuccin's userstyle contributing guidelines.
• I used AI (or AI-assistance) for this change.

5.6 sol ultra was used.

🔧 What does this fix? 🔧

Sets ChatGPT's --theme-user-msg-text custom property to Catppuccin's @text color beside the existing user-message background variable. This prevents ChatGPT's upstream default from rendering user messages with black text.

Closes #2298

🔍 Reproduction Steps 🔍

1. Enable the Catppuccin userstyle on https://chatgpt.com.
2. Open a conversation containing a user message.
3. Confirm that the user-message text uses the selected Catppuccin flavor's text color.

✅ Verification

• deno fmt --check styles/chatgpt/catppuccin.user.less
• deno task lint styles/chatgpt
• deno task lint
• deno task test — 7 passed

[Soontosh]

Thank you @JosephDemarest !