soften nav background so it blends with the page (#14)

Per Grant (channel, 2026-06-01): the top nav was pure #ffffff and stood
out as a hard band against the rest of the page, which carries subtle
radial-gradient atmospherics on top of #ffffff. Stepping it down one
notch to zinc-100 (#f4f4f5) puts it slightly under the body baseline so
the strip reads as part of the page rather than a chrome overlay. Dark
mode left untouched (already zinc-950).

Co-authored-by: Claude Opus 4.7 (1M context) <[email protected]>

Author: ross-makeacompany

frontend/src/components/SiteNav.tsx

@@ -18,7 +18,7 @@ export function SiteNav() {
   return (
     <nav
       aria-label="Site navigation"
-      className="fixed inset-x-0 top-0 z-50 flex items-center justify-between bg-white px-4 py-1.5 dark:bg-zinc-950 md:px-6 print:hidden"
+      className="fixed inset-x-0 top-0 z-50 flex items-center justify-between bg-zinc-100 px-4 py-1.5 dark:bg-zinc-950 md:px-6 print:hidden"
     >
       <div>
         {pathname === "/resume" && (