Move shakapacker to devDependencies in Pro dummy package.json

Shakapacker is a build-time tool and should be in devDependencies, not dependencies.

Changed in react_on_rails_pro/spec/dummy/package.json:
- Removed "shakapacker": "9.3.0" from dependencies
- Added "shakapacker": "9.3.0" to devDependencies

This completes the correct categorization of all build-time tools
(@swc/core, swc-loader, and shakapacker) as devDependencies.

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

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

Author: justin808

react_on_rails_pro/spec/dummy/package.json

@@ -63,7 +63,6 @@
     "sass": "^1.43.4",
     "sass-loader": "^12.3.0",
     "sass-resources-loader": "^2.0.0",
-    "shakapacker": "9.3.0",
     "style-loader": "^3.3.1",
     "tailwindcss": "^3.2.7",
     "terser-webpack-plugin": "5",
@@ -89,6 +88,7 @@
     "jsdom": "^16.4.0",
     "pino-pretty": "^13.0.0",
     "preload-webpack-plugin": "^3.0.0-alpha.1",
+    "shakapacker": "9.3.0",
     "swc-loader": "^0.2.6",
     "typescript": "^5.2.2",
     "webpack-dev-server": "^4.7.3"