fix: Bibliothek section always visible in left sidebar

- LeftSidebar was hiding entire Bibliothek section when blocks array was empty
- Removed blocks.length > 0 conditional wrapper
- Added blocks || [] fallback to prevent crash
- Library tree now visible even with no blocks

frontend/src/components/LeftSidebar.tsx

@@ -121,11 +121,10 @@ const LeftSidebar: React.FC<LeftSidebarProps> = ({ activeTool, onToolChange, sel
         </div>
       )}
 
-      {blocks && blocks.length > 0 && (
       <div className="tool-section" key="library">
         <div className="tool-section-label">Bibliothek</div>
           <BlockLibrary
-            blocks={blocks}
+            blocks={blocks || []}
             category="Alle"
             onCategoryChange={onBlockCategoryChange ?? (() => {})}
             onSearch={onBlockSearch ?? (() => {})}
@@ -137,7 +136,6 @@ const LeftSidebar: React.FC<LeftSidebarProps> = ({ activeTool, onToolChange, sel
             onSaveGroupAsBlock={onSaveGroupAsBlock}
           />
       </div>
-      )}
 
       <div className="leftbar-footer">
         {onToggleCollapse && (