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/icon.svg

@@ -0,0 +1,8 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 32 32">
+  <!-- Dark rounded background matching app --color-bg -->
+  <rect width="32" height="32" rx="6" fill="#0c1220"/>
+  <!-- Amber T: horizontal bar -->
+  <rect x="6" y="8" width="20" height="4" rx="1.5" fill="#f59e0b"/>
+  <!-- Amber T: vertical stem -->
+  <rect x="13" y="12" width="6" height="14" rx="1.5" fill="#f59e0b"/>
+</svg>