refactor(modals): split directory modal into separate create board modal

Separate DirectoryModal (for home view placeholders) and CreateBoardModal (for board view create button) into distinct components with their own stores, allowing independent styling and cleaner separation of concerns.

src/views/home/boards-list/boards-list.tsx

@@ -49,7 +49,7 @@ const BoardsList = ({ multisub }: { multisub: MultisubSubplebbit[] }) => {
   // Handler for placeholder board links
   const handlePlaceholderClick = (e: React.MouseEvent<HTMLAnchorElement>) => {
     e.preventDefault();
-    openDirectoryModal('placeholder');
+    openDirectoryModal();
   };
 
   if (loading) {