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/hooks/useCanvasStore.ts

@@ -15,7 +15,7 @@ interface CanvasState {
   onNodesChange: (changes: NodeChange[]) => void;
   onEdgesChange: (changes: EdgeChange[]) => void;
   onConnect: (connection: Connection) => void;
-  addNode: (type: NodeType, position: { x: number; y: number }) => void;
+  addNode: (type: NodeType, position: { x: number; y: number }, data?: Partial<CanvasNodeData>) => void;
   updateNodeData: (nodeId: string, data: Partial<CanvasNodeData>) => void;
   selectNode: (nodeId: string | null) => void;
   selectEdge: (edgeId: string | null) => void;
@@ -112,12 +112,14 @@ export const useCanvasStore = create<CanvasState>((set, get) => ({
     set({ edges: addEdge(connection, get().edges) });
   },
 
-  addNode: (type, position) => {
+  addNode: (type, position, customData) => {
     const newNode: Node<CanvasNodeData> = {
       id: getNodeId(),
       type,
       position,
-      data: getDefaultData(type),
+      data: customData
+        ? { ...getDefaultData(type), ...customData } as CanvasNodeData
+        : getDefaultData(type),
     };
     set({ nodes: [...get().nodes, newNode] });
   },