Advanced Theme Creation: AI, Manual, and Forking

[claudemyburgh]

This PR introduces several new ways for users to create themes, making the platform much more flexible and interactive.

Key Features:

• AI Theme Generation: Users can now describe a theme in plain English, and the AI will generate a complete set of colors and metadata.
• Manual Theme Editor: A new tabbed interface allows users to manually specify title, slug, description, and every CSS color variable for both light and dark modes.
• Advanced Color Picker: The custom color picker supports standard hex input and includes a comprehensive Tailwind CSS color selector.
• Theme Forking: Users can now click "Fork Theme" on any existing theme to use it as a base for their own custom creations.
• Real-time Preview: As users edit colors or generate them via AI, a live preview card updates instantly to show how the theme looks.

Technical Changes:

• Backend: AiService was enhanced with generateFullTheme. ThemesController was updated to handle multiple creation paths. A new ThemesControllerTest covers the logic.
• Frontend: New components were created for the creation flow. The create.tsx page was completely redesigned with a tabbed layout.
• Utility: buildCSSVars was improved to automatically wrap raw HSL values from the editor in hsl() functions for correct browser rendering.

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PR created automatically by Jules for task 2953705128562490666 started by @claudemyburgh

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