Fix theming and spacing: alpha-value colors, remove v4 CSS syntax, clean card padding

- tailwind.config.ts: add <alpha-value> to all color definitions so opacity
  modifiers (ring-foreground/10, outline-ring/50) render correctly
- globals.css: move @keyframes out of @theme inline block (v4-only, ignored
  by browser in v3); remove @custom-variant block (also v4-only)
- page.tsx, experience/page.tsx, publications/page.tsx: remove pb-3 from
  CardHeaders and pt-0 from CardContents — new shadcn card uses gap-4 so
  these overrides added extra space instead of removing it

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/app/publications/page.tsx

@@ -146,7 +146,7 @@ export default function PublicationsPage() {
                     </div>
                   )}
                 </CardHeader>
-                <CardContent className="pt-0">
+                <CardContent>
                   {pub.abstract && (
                     <p className="break-words text-sm leading-relaxed text-muted-foreground">
                       {pub.abstract}