fix: address additional code review feedback

- Add path.normalize() for Windows mixed separator handling
- Add validation to check Node executable exists after finding it
- Improve error dialog with specific troubleshooting advice for Node.js
  related errors vs general errors
- Include source info in validation error message

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

libs/platform/src/node-finder.ts

@@ -334,8 +334,10 @@ export function buildEnhancedPath(nodePath: string, currentPath: string = ''): s
 
   // Don't add if already present or if it's just '.'
   // Use path segment matching to avoid false positives (e.g., /opt/node vs /opt/node-v18)
-  const pathSegments = currentPath.split(path.delimiter);
-  if (nodeDir === '.' || pathSegments.includes(nodeDir)) {
+  // Normalize paths for comparison to handle mixed separators on Windows
+  const normalizedNodeDir = path.normalize(nodeDir);
+  const pathSegments = currentPath.split(path.delimiter).map((s) => path.normalize(s));
+  if (normalizedNodeDir === '.' || pathSegments.includes(normalizedNodeDir)) {
     return currentPath;
   }