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/LeftSidebar.tsx

@@ -56,12 +56,12 @@ const sections: Array<{ label: string; tools: ToolDef[] }> = [
   { label: 'Bestuhlung', tools: sectionBestuhlung },
 ];
 
-const LeftSidebar: React.FC<LeftSidebarProps> = ({ activeTool, onToolChange, selectedTemplate, onTemplateSelect }) => {
+const LeftSidebar: React.FC<LeftSidebarProps> = ({ activeTool, onToolChange, selectedTemplate, onTemplateSelect, onCollapse }) => {
   return (
     <aside className="leftbar" aria-label="Werkzeugpalette">
       <div className="leftbar-header">
         <span className="leftbar-title">Werkzeuge</span>
-        <button className="leftbar-toggle" aria-label="Linke Leiste ausblenden">
+        <button className="leftbar-toggle" aria-label="Linke Leiste ausblenden" onClick={onCollapse}>
           <svg viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round"><polyline points="15 18 9 12 15 6"/></svg>
         </button>
       </div>