fix: fit landing card previews to a real stage instead of a flat scale

[starc007]

What

Landing cards flat-scaled every preview by 0.8, so anything with a bigger natural footprint (or a w-full layout with no definite width to resolve against) got clipped or collapsed into a narrow "mobile" column.

• New PreviewFit: renders each preview at a fixed 460px stage (so w-full/flex-wrap layouts resolve exactly like they do on the component's own detail page), measures its natural height, and scales the whole stage to fit the card — still capped at the original 0.8 (0.84 on hover) for normal-size previews.
• Fixed a measurement bug along the way: the old fit math read getBoundingClientRect() on the same element the scale transform was applied to, which reflects the post-transform box and compounded into the wrong scale every render.
• Autoplay the shader-background preview (cycles variants every 2.4s, pauses on hover/focus, respects reduced motion) instead of sitting frozen on one variant.
• Stop stretching short testimonial cards to match a taller neighbor's height (h-full in a marquee row).

Test plan

• bun run check clean (typecheck, lint, registry — 42 components)