Fix dead code, isEditing guard, and empty state condition

- Remove unused hashCode function from markdown-text.tsx (leftover from
  removed artifacts feature)
- Include editing state in isEditing check so Cancel button stays visible
  when user clears name/content fields while editing an existing prompt
- Fix empty state message to only show when no items exist in any section
  (pinned, folders, or unfiled), not just when filteredItems is empty

studio/frontend/src/components/assistant-ui/markdown-text.tsx

@@ -50,15 +50,6 @@ type CodeFence = {
   source: string;
 };
 
-function hashCode(str: string): string {
-  let hash = 0;
-  for (let i = 0; i < str.length; i++) {
-    const char = str.charCodeAt(i);
-    hash = ((hash << 5) - hash + char) | 0;
-  }
-  return Math.abs(hash).toString(36);
-}
-
 function getMermaidSource(blockContent: string): string | null {
   const source = blockContent.match(MERMAID_SOURCE_RE)?.[1]?.trim();
   return source && source.length > 0 ? source : null;

studio/frontend/src/features/chat/components/prompt-library-sheet.tsx

@@ -88,7 +88,7 @@ export const PromptLibrarySheet: FC<{
   );
 
   const items = prompts ?? [];
-  const isEditing = name !== "" || content !== "";
+  const isEditing = editing !== null || name !== "" || content !== "";
 
   return (
     <Sheet open={open} onOpenChange={onOpenChange}>

studio/frontend/src/features/chat/thread-sidebar.tsx

@@ -448,7 +448,7 @@ export function ThreadSidebar({
             <SidebarMenu>
               {unfiledItems.map(renderItem)}
             </SidebarMenu>
-            {filteredItems.length === 0 && (
+            {unfiledItems.length === 0 && pinnedItems.length === 0 && folderedItems.size === 0 && (
               <p className="px-2 py-6 text-center text-xs text-muted-foreground">
                 {debouncedQuery.trim() ? "No matching threads" : "No threads yet"}
               </p>