feat(design): external link colors, section dividers, nav scroll-spy

Dusty blue/violet link colors for external links (target="_blank") with
visited state. Both dark and light theme tokens. Mailto links included.

Section dividers via border-top on anchored sections. Scroll-margin-top
clears sticky nav on anchor jumps.

Nav scroll-spy highlights active section using IntersectionObserver +
scroll listener fallback for bottom-of-page edge case.

src/components/ProjectCard.astro

@@ -15,7 +15,7 @@ const { project } = Astro.props;
         href={project.githubUrl}
         target="_blank"
         rel="noopener noreferrer"
-        class="font-semibold underline hover:text-[var(--color-text-label)]"
+        class="font-semibold underline"
       >
         {project.title}
       </a>