feat: add --zen-dialog-background for proper dialog theming

[salmonumbrella]

Summary

• Add --zen-dialog-background CSS variable to the userChrome.tera template
• Regenerate all theme variants with the new variable

This fixes dialogs (like History popup, Clear cookies modal, and other subdialogs) not receiving Catppuccin colors. These UI elements were appearing in light mode because Zen Browser uses --zen-dialog-background for the canvas background of dialog boxes.

What's fixed

• History popup window
• Clear cookies and site data modal
• Other browser subdialogs and panels that use --background-color-canvas

Technical details

Zen Browser's zen-theme.css defines:

--zen-dialog-background: light-dark(rgb(244, 244, 244), rgb(31, 31, 31));

This variable is then used for:

• --background-color-canvas (dialog root elements)
• --arrowpanel-background (already set by Catppuccin theme)

Without setting --zen-dialog-background, dialogs that inherit from --background-color-canvas fall back to Zen's default colors instead of the Catppuccin palette.

Test plan

• Open History popup (Ctrl+Shift+H or History menu)
• Open Clear cookies modal (Settings → Privacy → Clear Data)
• Verify both have Catppuccin background colors instead of light/default theme

🤖 Generated with Claude Code