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/backend/api/routes/nx.py

@@ -0,0 +1,90 @@
+"""
+NX API Routes
+
+Provides NX model introspection capabilities for the Canvas Builder.
+"""
+
+from fastapi import APIRouter, HTTPException
+from pydantic import BaseModel
+from typing import Optional
+
+router = APIRouter()
+
+
+class IntrospectRequest(BaseModel):
+    file_path: str
+
+
+@router.post("/introspect")
+async def introspect_model(request: IntrospectRequest):
+    """
+    Introspect an NX model file to discover expressions, solver type, and dependencies.
+
+    Args:
+        file_path: Relative path from studies root (e.g., "M1_Mirror/study_v1/model.sim")
+
+    Returns:
+        Introspection result with expressions, solver_type, dependent_files, extractors
+    """
+    try:
+        from api.services.nx_introspection import NXIntrospector
+
+        introspector = NXIntrospector(request.file_path)
+        result = introspector.introspect()
+        return result
+    except Exception as e:
+        raise HTTPException(status_code=500, detail=str(e))
+
+
+@router.get("/expressions")
+async def get_expressions(file_path: str):
+    """
+    Get expressions from an NX model.
+
+    Args:
+        file_path: Relative path from studies root
+
+    Returns:
+        List of expressions with names, values, units
+    """
+    try:
+        from api.services.nx_introspection import NXIntrospector
+
+        introspector = NXIntrospector(file_path)
+        result = introspector.introspect()
+        return {
+            "expressions": result.get("expressions", []),
+            "file_path": file_path,
+            "source": "introspection",
+        }
+    except Exception as e:
+        raise HTTPException(status_code=500, detail=str(e))
+
+
+@router.get("/extractors")
+async def list_extractors(solver_type: Optional[str] = None):
+    """
+    List available extractors, optionally filtered by solver type.
+
+    Args:
+        solver_type: Optional solver type (SOL101, SOL103, etc.)
+
+    Returns:
+        List of available extractors with their descriptions
+    """
+    from api.services.nx_introspection import NXIntrospector
+
+    # Create a dummy introspector to get extractor suggestions
+    class DummyIntrospector:
+        def __init__(self):
+            self.parent_dir = ""
+
+    dummy = NXIntrospector.__new__(NXIntrospector)
+    dummy.parent_dir = ""
+
+    extractors = dummy._suggest_extractors(solver_type)
+
+    return {
+        "extractors": extractors,
+        "solver_type": solver_type,
+    }