feat: add aurora and borealis themes

[arizen-dev]

Issue for this PR

Closes #

Type of change

• New feature

What does this PR do?

Adds two new built-in themes to OpenCode:

• Aurora — a clean light theme with subtle purple/blue accents inspired by the macOS color palette. Includes a complementary dark variant.
• Borealis — a dual-mode theme with a deep navy dark mode and a cool-toned light mode, designed for users who want a softer alternative to pure black/white themes.

Both themes follow the desktop-theme.json format and match the structure of existing built-in themes like nord and catppuccin.

How did you verify your code works?

• Theme JSON files follow the exact same schema as existing themes in packages/ui/src/theme/themes/
• Both themes include light and dark variants with complete palette + syntax override definitions
• Registered in default-themes.ts and exported from index.ts following the same pattern as other themes
• Themes were previously used as custom TUI themes (theme.json format) and confirmed working in production

Screenshots / recordings

N/A — theme files only, no UI code changes.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

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