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/Hero.astro

@@ -15,7 +15,7 @@
       href="https://github.com/lerko96"
       target="_blank"
       rel="noopener noreferrer"
-      class="underline hover:text-[var(--color-text)]"
+      class="underline"
     >
       GitHub
     </a>
@@ -23,7 +23,7 @@
       href="https://gitea.lerkolabs.com/lerko"
       target="_blank"
       rel="noopener noreferrer"
-      class="underline hover:text-[var(--color-text)]"
+      class="underline"
     >
       Gitea
     </a>
@@ -31,13 +31,14 @@
       href="https://www.linkedin.com/in/tyler-koenig"
       target="_blank"
       rel="noopener noreferrer"
-      class="underline hover:text-[var(--color-text)]"
+      class="underline"
     >
       LinkedIn
     </a>
     <a
       href="mailto:tyler@lerkolabs.com"
-      class="underline hover:text-[var(--color-text)]"
+      target="_blank"
+      class="underline"
     >
       Email
     </a>