feat(ui): add context-aware directory modal with different content for placeholder vs create board

Add modal context tracking to show different content when opened from placeholder board clicks vs create board button. Include information about Subscribe button and pubsub voting discovery pages. Update README with comprehensive explanation of 5chan's competitive directory system and value proposition.

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();
+    openDirectoryModal('placeholder');
   };
 
   if (loading) {