Add postinstall script to build shakapacker from GitHub branch

When installing shakapacker from a GitHub branch, the TypeScript source
needs to be compiled. Added a postinstall script that:
1. Detects if shakapacker was installed from source (has .ts but no .js files)
2. Installs shakapacker's dev dependencies
3. Runs the build script to compile TypeScript to JavaScript

This ensures the package is properly built even when installed directly
from GitHub instead of npm.

Also updated shakapacker reference to use explicit github: prefix for clarity.

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

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

Author: justin808

package.json

@@ -17,7 +17,7 @@
   },
   "homepage": "https://github.com/shakacode/react-webpack-rails-tutorial",
   "scripts": {
-    "postinstall": "patch-package",
+    "postinstall": "patch-package && if [ -f node_modules/shakapacker/package/index.ts ] && [ ! -f node_modules/shakapacker/package/index.js ]; then cd node_modules/shakapacker && yarn install --production=false && yarn build; fi",
     "res:clean": "rescript clean",
     "res:format": "rescript format -all",
     "res:dev": "yarn res:clean && rescript build -w",
@@ -92,7 +92,7 @@
     "sass": "^1.58.3",
     "sass-loader": "^13.3.2",
     "sass-resources-loader": "^2.2.5",
-    "shakapacker": "shakacode/shakapacker#justin808/early-hints",
+    "shakapacker": "github:shakacode/shakapacker#justin808/early-hints",
     "stimulus": "^3.0.1",
     "style-loader": "^3.3.1",
     "swc-loader": "^0.2.6",

yarn.lock

@@ -9135,7 +9135,7 @@ setprototypeof@1.2.0:
   resolved "https://registry.npmjs.org/setprototypeof/-/setprototypeof-1.2.0.tgz"
   integrity sha512-E5LDX7Wrp85Kil5bhZv46j8jOeboKq5JMmYM3gVGdGH8xFpPWXUMsNrlODCrkoxMEeNi/XZIwuRvY4XNwYMJpw==
 
-shakapacker@shakacode/shakapacker#justin808/early-hints:
+"shakapacker@github:shakacode/shakapacker#justin808/early-hints":
   version "9.3.0-beta.2"
   resolved "https://codeload.github.com/shakacode/shakapacker/tar.gz/6bf6bf5c21388a402362c516370606529d6cfecd"
   dependencies: