Optimized Rolling Rainbow Foil

Objective

Update the foil effect to use a "continuous rainbow" and a "milder white background" as specifically requested.

Modified FoilOverlay in src/client/src/components/CardPreview.tsx:
- Continuous Rainbow: Updated the gradient to encompass the full spectrum (red -> yellow -> green -> blue -> purple -> red) over a linear gradient. This seamless loop ensures the rolling animation (animate-bg-roll) is smooth and continuously colorful.
- Milder White Background: Removed the heavy soft-light and pulse layers. Replaced them with a very subtle white/5 overlay using mix-blend-overlay. This brightens the foil slightly without washing out the colors.
- Color Dodge: Applied mix-blend-color-dodge to the container to ensure the rainbow colors interact vibrantly with the underlying card art.

Foil cards now feature a smooth, full-spectrum rainbow scrolling effect that feels high-quality and "magical," with a balanced brightness level.