Add ReScript build step before starting development server

When bin/dev starts, webpack immediately tries to build bundles that
import ReScript components (HelloWorldReScript.res.js). However, these
.res.js files are generated by the ReScript compiler from .res source
files, and webpack fails if they don't exist yet.

Solution: Build ReScript files before starting the Procfile processes.
This ensures all .res.js files are generated before webpack starts.

Changes:
- lib/react_on_rails/dev/server_manager.rb:
  - Add build_rescript_if_needed method to compile ReScript before dev server
  - Check for both bsconfig.json (old) and rescript.json (new) config files
  - Run yarn build:rescript if ReScript is configured
  - Exit with error if ReScript build fails

Fixes webpack Module not found errors for HelloWorldReScript.res.js

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

Co-Authored-By: Claude <[email protected]>

Author: justin808

lib/react_on_rails/dev/server_manager.rb

@@ -416,18 +416,34 @@ def run_static_development(procfile, verbose: false, route: nil)
             route: route
           )
 
+          build_rescript_if_needed
           PackGenerator.generate(verbose: verbose)
           ProcessManager.ensure_procfile(procfile)
           ProcessManager.run_with_process_manager(procfile)
         end
 
         def run_development(procfile, verbose: false, route: nil)
           print_procfile_info(procfile, route: route)
+          build_rescript_if_needed
           PackGenerator.generate(verbose: verbose)
           ProcessManager.ensure_procfile(procfile)
           ProcessManager.run_with_process_manager(procfile)
         end
 
+        def build_rescript_if_needed
+          # Check for both old (bsconfig.json) and new (rescript.json) config files
+          return unless File.exist?("bsconfig.json") || File.exist?("rescript.json")
+
+          print "🔧 Building ReScript... "
+          success = system("yarn build:rescript > /dev/null 2>&1")
+          puts success ? "✅" : "❌"
+
+          return if success
+
+          puts "❌ ReScript build failed"
+          exit 1
+        end
+
         def print_server_info(title, features, port = 3000, route: nil)
           puts title
           features.each { |feature| puts "   - #{feature}" }