refactor: Colocate service worker build script with source files

Move build script from scripts/build-service-worker.js to
app/javascript/service-worker/build.js to keep all service worker
related code in one place.

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

package.json

@@ -13,7 +13,7 @@
     "dev": "overmind start -f ./Procfile.dev",
     "ruby:prettier": "bundle exec rubocop -a",
     "build:sdk": "BUILD_MODE=library vite build",
-    "build:sw": "node scripts/build-service-worker.js",
+    "build:sw": "node app/javascript/service-worker/build.js",
     "prepare": "husky install",
     "size": "size-limit",
     "story:dev": "histoire dev",