fix: valid config.json + content-hashed bundle (cache-bust)

- BuildConfig.sh: ${DEBUG:false} -> ${DEBUG:-false} and normalize to
  true/false. The old syntax produced empty -> invalid config.json
  ("DEBUG": }) when DEBUG was unset, breaking the whole frontend.
- webpack: output [name].[contenthash].js so index.html references a
  unique bundle URL each build; eliminates stale CDN/browser bundle
  after deploys (no manual cache purge needed).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

webpack.config.js

@@ -5,7 +5,11 @@ module.exports = {
     entry: './src/index.js',
     output: {
         path: path.resolve(__dirname, './dist'),
-        filename: 'bundle.js',
+        // Content-hashed filename: index.html (injected by HtmlWebpackPlugin,
+        // and not edge-cached) points at a unique URL each build, so a new
+        // deploy is picked up immediately — no stale CDN/browser bundle.
+        filename: '[name].[contenthash].js',
+        chunkFilename: '[name].[contenthash].js',
         publicPath: '/',
         clean: true,
     },