Full theme configuration system with shareable profiles + live pop-out preview

Summary

Full theme customization system for Little Genius. Major rewrite.

Scope

• Complete theme config: colors, layout, whatever surfaces make sense — full customization, not a few toggles.
• Shareable themes — export/import or share-code so users can send configs to each other.
• Pop-out preview window: lets user tweak theme while using the app live, so they can see if a change fits before committing.
• Multiple profiles: users can save/switch between several theme configs, not just one active theme.
• Current app appearance becomes the "Default" profile — baseline, not lost on rollout.

Notes

Large rewrite of theming layer. No implementation details finalized — architecture TBD.