Restructure share routes to fix client-side navigation

- Change from [...shareId] catch-all to [slug]/[[...path]] structure
- Layout with ShareProvider now persists across sub-route navigation
- Add slug to ShareData context (separate from shareId UUID)
- Links now use slug instead of UUID for proper routing
- Remove unused ShareFooter and ShareHeader files

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/app/share/[slug]/[[...path]]/page.tsx

@@ -0,0 +1,5 @@
+import { SharePage } from './SharePage';
+
+export default function () {
+  return <SharePage />;
+}