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/files.py

@@ -0,0 +1,155 @@
+"""
+Files API Routes
+
+Provides file browsing capabilities for the Canvas Builder.
+"""
+
+from fastapi import APIRouter, Query
+from pathlib import Path
+from typing import List, Optional
+import os
+
+router = APIRouter()
+
+# Path to studies root (go up 5 levels from this file)
+_file_path = os.path.abspath(__file__)
+ATOMIZER_ROOT = Path(os.path.normpath(os.path.dirname(os.path.dirname(os.path.dirname(
+    os.path.dirname(os.path.dirname(_file_path))
+)))))
+STUDIES_ROOT = ATOMIZER_ROOT / "studies"
+
+
+@router.get("/list")
+async def list_files(
+    path: str = "",
+    types: str = ".sim,.prt,.fem,.afem"
+):
+    """
+    List files in a directory, filtered by type.
+
+    Args:
+        path: Relative path from studies root (empty for root)
+        types: Comma-separated list of file extensions to include
+
+    Returns:
+        List of files and directories with their paths
+    """
+    allowed_types = [t.strip().lower() for t in types.split(',') if t.strip()]
+
+    base_path = STUDIES_ROOT / path if path else STUDIES_ROOT
+
+    if not base_path.exists():
+        return {"files": [], "path": path, "error": "Directory not found"}
+
+    files = []
+
+    try:
+        for entry in sorted(base_path.iterdir(), key=lambda x: (not x.is_dir(), x.name.lower())):
+            # Skip hidden files and directories
+            if entry.name.startswith('.'):
+                continue
+
+            if entry.is_dir():
+                # Include directories
+                files.append({
+                    "name": entry.name,
+                    "path": str(entry.relative_to(STUDIES_ROOT)).replace("\\", "/"),
+                    "isDirectory": True,
+                })
+            else:
+                # Include files matching type filter
+                suffix = entry.suffix.lower()
+                if suffix in allowed_types:
+                    files.append({
+                        "name": entry.name,
+                        "path": str(entry.relative_to(STUDIES_ROOT)).replace("\\", "/"),
+                        "isDirectory": False,
+                        "size": entry.stat().st_size,
+                    })
+    except PermissionError:
+        return {"files": [], "path": path, "error": "Permission denied"}
+    except Exception as e:
+        return {"files": [], "path": path, "error": str(e)}
+
+    return {"files": files, "path": path}
+
+
+@router.get("/search")
+async def search_files(
+    query: str,
+    types: str = ".sim,.prt,.fem,.afem",
+    max_results: int = 50
+):
+    """
+    Search for files by name pattern.
+
+    Args:
+        query: Search pattern (partial name match)
+        types: Comma-separated list of file extensions to include
+        max_results: Maximum number of results to return
+
+    Returns:
+        List of matching files with their paths
+    """
+    allowed_types = [t.strip().lower() for t in types.split(',') if t.strip()]
+    query_lower = query.lower()
+
+    files = []
+
+    def search_recursive(directory: Path, depth: int = 0):
+        """Recursively search for matching files"""
+        if depth > 10 or len(files) >= max_results:  # Limit depth and results
+            return
+
+        try:
+            for entry in directory.iterdir():
+                if len(files) >= max_results:
+                    return
+
+                if entry.name.startswith('.'):
+                    continue
+
+                if entry.is_dir():
+                    search_recursive(entry, depth + 1)
+                elif entry.suffix.lower() in allowed_types:
+                    if query_lower in entry.name.lower():
+                        files.append({
+                            "name": entry.name,
+                            "path": str(entry.relative_to(STUDIES_ROOT)).replace("\\", "/"),
+                            "isDirectory": False,
+                            "size": entry.stat().st_size,
+                        })
+        except (PermissionError, OSError):
+            pass
+
+    search_recursive(STUDIES_ROOT)
+
+    return {"files": files, "query": query, "total": len(files)}
+
+
+@router.get("/exists")
+async def check_file_exists(path: str):
+    """
+    Check if a file exists.
+
+    Args:
+        path: Relative path from studies root
+
+    Returns:
+        Boolean indicating if file exists and file info
+    """
+    file_path = STUDIES_ROOT / path
+    exists = file_path.exists()
+
+    result = {
+        "exists": exists,
+        "path": path,
+    }
+
+    if exists:
+        result["isDirectory"] = file_path.is_dir()
+        if file_path.is_file():
+            result["size"] = file_path.stat().st_size
+            result["name"] = file_path.name
+
+    return result