feat: click-drag drawing, layer tree, settings modal, UI improvements

- interaction: click-drag drawing for 2-point tools (line, rect, circle, dimension, leader)
- interaction: preview rendering during drag with setPreviewElement
- interaction: handleDrawDown only sets phase=drawing on first point
- CanvasArea: sync activeLayerId to LayerManager
- CanvasArea: callback refresh useEffect prevents stale closures
- App.tsx: functional setElements updates for handleElementCreated/Deleted/Modified
- LayerPanel: tree structure with sub-layers (onReorder, onAddSubLayer)
- TreeView: drag-and-drop reordering support
- RightSidebar: onReorder, onAddSubLayer, onUpdateElement props
- PropertiesPanel: onUpdateElement prop
- SettingsModal: personal/password/language/theme/users/plugins/AI tabs
- styles.css: extensive UI styling additions
- types: updated RightSidebarProps, CanvasAreaProps, LayerPanelProps

frontend/src/components/BlockLibrary.tsx

@@ -88,7 +88,7 @@ const BlockLibrary: React.FC<BlockLibraryProps> = ({ blocks, category, onCategor
       </div>
       <div className="tree tree-library" role="tree" aria-label="Bibliothek-Struktur">
         {Object.entries(grouped).map(([cat, catBlocks]) => (
-          <div key={cat} className="tree-node">
+          <div key={cat} className="lib-tree-node">
             <div className="tree-item tree-item-folder" onClick={() => toggleCat(cat)}>
               <span className="tree-toggle">{expandedCats.has(cat) ? '▾' : '▸'}</span>
               <span className="tree-icon">📁</span>