feat(canvas): Add file browser, introspection, and improve node flow

Phase 1-7 of Canvas V4 Ralph Loop implementation:

Backend:
- Add /api/files routes for browsing model files
- Add /api/nx routes for NX model introspection
- Add NXIntrospector service to discover expressions and extractors
- Add health check with database status

Frontend:
- Add FileBrowser component for selecting .sim/.prt/.fem files
- Add IntrospectionPanel to discover expressions and extractors
- Update NodeConfigPanel with browse and introspect buttons
- Update schema with NODE_HANDLES for proper flow direction
- Update validation for correct DesignVar -> Model -> Solver flow
- Update useCanvasStore.addNode() to accept custom data

Flow correction: Design Variables now connect TO Model (as source),
not FROM Model. This matches the actual data flow in optimization.

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

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

@@ -1,5 +1,9 @@
+import { useState } from 'react';
+import { FolderSearch, Microscope } from 'lucide-react';
 import { useCanvasStore } from '../../../hooks/useCanvasStore';
 import { ExpressionSelector } from './ExpressionSelector';
+import { FileBrowser } from './FileBrowser';
+import { IntrospectionPanel } from './IntrospectionPanel';
 import {
   ModelNodeData,
   SolverNodeData,
@@ -24,6 +28,9 @@ export function NodeConfigPanel({ nodeId }: NodeConfigPanelProps) {
   const { nodes, updateNodeData, deleteSelected } = useCanvasStore();
   const node = nodes.find((n) => n.id === nodeId);
 
+  const [showFileBrowser, setShowFileBrowser] = useState(false);
+  const [showIntrospection, setShowIntrospection] = useState(false);
+
   if (!node) return null;
 
   const { data } = node;
@@ -63,15 +70,24 @@ export function NodeConfigPanel({ nodeId }: NodeConfigPanelProps) {
           <>
             <div>
               <label className={labelClass}>
-                File Path
+                Model File
               </label>
-              <input
-                type="text"
-                value={(data as ModelNodeData).filePath || ''}
-                onChange={(e) => handleChange('filePath', e.target.value)}
-                placeholder="path/to/model.prt"
-                className={`${inputClass} font-mono text-sm`}
-              />
+              <div className="flex gap-2">
+                <input
+                  type="text"
+                  value={(data as ModelNodeData).filePath || ''}
+                  onChange={(e) => handleChange('filePath', e.target.value)}
+                  placeholder="path/to/model.sim"
+                  className={`${inputClass} font-mono text-sm flex-1`}
+                />
+                <button
+                  onClick={() => setShowFileBrowser(true)}
+                  className="px-3 py-2 bg-dark-700 hover:bg-dark-600 rounded-lg text-dark-300 hover:text-white transition-colors"
+                  title="Browse files"
+                >
+                  <FolderSearch size={18} />
+                </button>
+              </div>
             </div>
             <div>
               <label className={labelClass}>
@@ -86,8 +102,21 @@ export function NodeConfigPanel({ nodeId }: NodeConfigPanelProps) {
                 <option value="prt">Part (.prt)</option>
                 <option value="fem">FEM (.fem)</option>
                 <option value="sim">Simulation (.sim)</option>
+                <option value="afem">Assembled FEM (.afem)</option>
               </select>
             </div>
+            {/* Introspect Button */}
+            {(data as ModelNodeData).filePath && (
+              <button
+                onClick={() => setShowIntrospection(true)}
+                className="w-full flex items-center justify-center gap-2 px-3 py-2.5 bg-primary-500/20
+                           hover:bg-primary-500/30 border border-primary-500/30 rounded-lg
+                           text-primary-400 text-sm font-medium transition-colors"
+              >
+                <Microscope size={16} />
+                Introspect Model
+              </button>
+            )}
           </>
         )}
 
@@ -385,6 +414,27 @@ export function NodeConfigPanel({ nodeId }: NodeConfigPanelProps) {
           </>
         )}
       </div>
+
+      {/* File Browser Modal */}
+      <FileBrowser
+        isOpen={showFileBrowser}
+        onClose={() => setShowFileBrowser(false)}
+        onSelect={(path, fileType) => {
+          handleChange('filePath', path);
+          handleChange('fileType', fileType.replace('.', ''));
+        }}
+        fileTypes={['.sim', '.prt', '.fem', '.afem']}
+      />
+
+      {/* Introspection Panel */}
+      {showIntrospection && (data as ModelNodeData).filePath && (
+        <div className="fixed top-20 right-96 z-40">
+          <IntrospectionPanel
+            filePath={(data as ModelNodeData).filePath!}
+            onClose={() => setShowIntrospection(false)}
+          />
+        </div>
+      )}
     </div>
   );
 }