fix: add cross-platform Node.js executable finder for desktop launches

When the Electron app is launched from desktop environments (macOS Finder,
Windows Explorer, Linux desktop launchers), the PATH environment variable
is often limited and doesn't include Node.js installation paths.

This adds a new `findNodeExecutable()` utility to @automaker/platform that:
- Searches common installation paths (Homebrew, system, Program Files)
- Supports version managers: NVM, fnm, nvm-windows, Scoop, Chocolatey
- Falls back to shell resolution (which/where) when available
- Enhances PATH for child processes via `buildEnhancedPath()`
- Works cross-platform: macOS, Windows, and Linux

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

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

libs/platform/src/index.ts

@@ -44,3 +44,11 @@ export {
 
 // Secure file system (validates paths before I/O operations)
 export * as secureFs from './secure-fs.js';
+
+// Node.js executable finder (cross-platform)
+export {
+  findNodeExecutable,
+  buildEnhancedPath,
+  type NodeFinderResult,
+  type NodeFinderOptions,
+} from './node-finder.js';