Fix M-4: punctuation-only page name yields empty slug ('.html')

slugify('!!!') stripped down to '' since punctuation-only names have no
a-z0-9 characters left; buildSavePayload then wrote filename = '' +
'.html'. slugify now falls back to 'page' when the computed slug is
empty, so uniqueSlug's existing dedupe (page, page-2, ...) applies same
as any other collision. Landing page slug stays forced to 'index'.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

craft/src/state/PageContext.tsx

@@ -181,12 +181,17 @@ const PageContext = createContext<PageContextValue>({
 export const usePages = () => useContext(PageContext);
 
 function slugify(name: string): string {
-  return name
+  const slug = name
     .toLowerCase()
     .trim()
     .replace(/[^a-z0-9\s-]/g, '')
     .replace(/\s+/g, '-')
     .replace(/-+/g, '-');
+  // M-4: a punctuation-only name (e.g. "!!!") strips down to '' -- without a
+  // fallback, buildSavePayload would write filename = '' + '.html' for that
+  // page. uniqueSlug's existing dedupe logic then applies on top of this
+  // fallback the same way it does for any other base ('page', 'page-2', ...).
+  return slug || 'page';
 }
 
 /**