Fix transparent CTA and footer backgrounds

The design-system colour variables are RGB triplets (--ids__surface-RGB: 238, 240, 242) meant to be wrapped in rgb()/rgba(). Two components use them as a colour directly, so the browser discards the declaration and both blocks render with a transparent background. Computed style on production confirms rgba(0, 0, 0, 0) for both.

Current State

• src/lib/CallToAction.svelte — .cta has background: var(--ids__surface-RGB)
• src/routes/Footer.svelte — footer has background: var(--ids__surface-RGB) and mask: var(--squircle-mask); the mask variable is not defined anywhere

Tasks

• Wrap both in rgb(var(--ids__surface-RGB)), or better, add resolved colour tokens (--ids__surface) to colors.css and migrate away from the triplet pattern
• Define --squircle-mask or drop the mask declaration and use .ids__rounded
• Grep the rest of the codebase for other bare var(--ids__*-RGB) usages outside rgb()/rgba()