feat(canvas): Add ExpressionSelector for design variable configuration

Phase 5 of Canvas Professional Upgrade:
- Create ExpressionSelector component with searchable dropdown
- Fetch expressions from NX introspection API
- Auto-populate label, units, and default min/max from expression value
- Add refresh button to reload expressions
- Integrate into NodeConfigPanel for DesignVar nodes

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

atomizer-dashboard/frontend/src/components/canvas/panels/NodeConfigPanel.tsx

@@ -1,4 +1,5 @@
 import { useCanvasStore } from '../../../hooks/useCanvasStore';
+import { ExpressionSelector } from './ExpressionSelector';
 import {
   ModelNodeData,
   SolverNodeData,
@@ -117,12 +118,24 @@ export function NodeConfigPanel({ nodeId }: NodeConfigPanelProps) {
               <label className={labelClass}>
                 Expression Name
               </label>
-              <input
-                type="text"
+              <ExpressionSelector
                 value={(data as DesignVarNodeData).expressionName || ''}
-                onChange={(e) => handleChange('expressionName', e.target.value)}
-                placeholder="thickness"
-                className={`${inputClass} font-mono`}
+                onChange={(name, value, units) => {
+                  handleChange('expressionName', name);
+                  handleChange('label', name || 'Design Variable');
+                  if (units) handleChange('unit', units);
+                  // Set default min/max around current value
+                  if (value !== undefined) {
+                    const dvData = data as DesignVarNodeData;
+                    if (dvData.minValue === undefined) {
+                      handleChange('minValue', value * 0.5);
+                    }
+                    if (dvData.maxValue === undefined) {
+                      handleChange('maxValue', value * 1.5);
+                    }
+                  }
+                }}
+                placeholder="Select NX expression..."
               />
             </div>
             <div className="grid grid-cols-2 gap-2">