Add interactive gravity-well background mesh

Details

Render a full-viewport canvas grid of squares behind the v2 page that
warps toward the cursor like a mass on a taut sheet (gravity-well /
spacetime-warp effect, inspired by igloo.inc). Vertices are pulled with
a Gaussian falloff, capped to a fraction of the cursor distance so the
mesh dimples into a smooth bowl instead of a spiky pinch, and return to
rest on a light spring so it wobbles like a pinned knitted blanket.
Line colors are read live from the --v2-accent theme variable, so the
mesh blends with both the dark (orange) and light palettes; brightness
and width ramp up near the well to suggest depth. The page background
moves to the root element so the z-index:-1 canvas shows through the
transparent body, and the canvas is pointer-events:none so it never
intercepts clicks. Honors prefers-reduced-motion by drawing a static
grid with no animation loop, and idles the rAF loop once settled.

Files changed (3)