chore: rebrand to Thoosie Calendar, add favicon

- Rename app title and header from "Six Flags Calendar" to "Thoosie Calendar"
- Update layout metadata title and description
- Update README title
- Add app/icon.svg favicon: amber T on dark background, picked up
  automatically by Next.js App Router

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

app/page.tsx

@@ -84,7 +84,7 @@ export default async function HomePage({ searchParams }: PageProps) {
             color: "var(--color-text)",
             letterSpacing: "-0.02em",
           }}>
-            Six Flags Calendar
+            Thoosie Calendar
           </span>
 
           <span style={{