feat: port CursorGrid animation from react-bits

[adetyaz]

Summary

• Ports the Cursor Grid animation from react-bits (ts-tailwind variant) — a canvas grid where cells near the cursor light up with a falloff-based glow and fade out over time, with optional click pulses and a static lattice overlay.
• Preserves props, defaults, and behavior 1:1 with upstream, including the two-tier reactivity model: only cellSize changes rebuild the grid (canvas resize, ResizeObserver, listeners); every other visual prop (color, radius, falloff, opacities, etc.) repaints live without tearing anything down, mirroring react-bits' propsRef pattern via Svelte 5's automatic effect dependency tracking.
• Registers the component in categories.ts (Animations subcategory, IMPLEMENTED_DEMOS, NEW badge) and adds its docs/demo page with full customize controls and prop table.

Test plan

• npm run check — no errors/warnings in the new files
• npm run lint — clean
• npm run registry:build — jsrepo registry sync passes (categories.ts matches filesystem)
• npm run build — production build succeeds
• Manual verification in dev server: grid lights up under cursor, click pulses, sliders/color picker/falloff select/click-pulse switch all work, "Move Your Cursor" hint renders correctly in light/dark themes, "New" badge shows in sidebar nav