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/sections/EventList-Cards.tsx

@@ -199,7 +199,7 @@ export function Card({
   const igHandle = ev.instagram || null;
   const igUrl = igHandle
     ? `https://instagram.com/${igHandle.replace(/^@/, "")}`
-    : null;
+    : undefined;
 
   /* Off unless the caller says the band is mixed. A "Retreat" badge
      on every card in a row of nothing but retreats is noise, and