Fix the remaining type errors in src/

- Declare the --ig-fill custom property on React's CSSProperties once,
  instead of casting the style prop in Footer, Home and EventList-Cards.
- Type Footer's Get_In_Touch links as internal or external, so the
  external branch type-checks.
- Pass undefined rather than null for an absent gradient (Home) and
  Instagram URL (EventList-Cards).

tsc --noEmit is now clean, with and without --noImplicitAny.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/pages/Home.tsx

@@ -300,7 +300,7 @@ export default function Home() {
                     className="rounded-3xl text-black overflow-hidden shadow-2xl h-full"
                     style={{
                       border: `1px solid ${color}`,
-                      background: ev.gradient,
+                      background: ev.gradient ?? undefined,
                       filter: past ? "saturate(0.75)" : "none",
                       pointerEvents: active ? "auto" : "none",
                     }}