fix(spec-page): add null guard for specData to fix TypeScript build

TypeScript couldn't narrow specData after the error/loading early returns.
Added explicit null check before the main render to satisfy strict null checks.

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

Author: MarkusNeusinger

app/src/pages/SpecPage.tsx

@@ -209,6 +209,9 @@ export function SpecPage() {
     );
   }
 
+  // After loading/error/not-found guards, specData is guaranteed to be non-null
+  if (!specData) return null;
+
   return (
     <>
       <Helmet>