Redesign the editor's inspector and layer panel, add layer thumbnails

I redesigned the inspector and layer panel to bring a tighter, more consistent rhythm — property rows, labels, and switches now follow one shared layout instead of a patchwork of spacing from different features added over time. Sliders and switches got their frosted-glass look back, and row density is looser so the panel doesn’t feel cramped when you’re editing several properties at once.

The layer panel also shows a small thumbnail preview for each layer, so you can tell shapes apart at a glance instead of relying on names alone.