Personal Themes Feature

[claudemyburgh]

Authenticated users can now create and edit their own themes. The implementation includes:

• A new controller UserThemeController handling theme management.
• Frontend components ThemeForm and ThemePreview for a rich editing experience.
• Real-time contrast checking using culori to ensure accessibility.
• Automatic synchronization between standard Tailwind v4 color dropdowns and a color picker.
• Logic to set a theme as the account default.
• Feature tests for backend logic.

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PR created automatically by Jules for task 11589093623667468578 started by @claudemyburgh

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