Library detail panel on hover

[milanofthe]

• Block & Events Library: hover any tile to expand the panel with a detail column showing a canvas-accurate block preview, toolbox info, and the full RST documentation
• Open/switch/close delays prevent flicker when brushing past tiles on the way to the detail column
• ResizablePanel learns an optional rightColumn snippet (gated by rightColumnActive) so other panels remain untouched
• Footer spans both columns via the existing footer slot; header treatment unchanged
• Pre-existing svelte-check error (fit prop) and warning (dragPreviewElement not $state) cleaned up — repo is now 0 errors / 0 warnings