chore: turn sourcemaps back on

Ok, parent commit ran without errors:

- https://github.com/wincent/masochist/actions/runs/10384681229/job/28752223953

```
Starting upload with concurrency 20.
Will look for sourcemaps in dist
Will match JS files for errors on files starting with /static/
version: 9764ba1 service: masochist project path: /./

Command summary:
⚠️ No sourcemaps detected. Did you specify the correct directory?
```

So, we're ready for phase 2. Let's turn sourcemaps back on, having
previously turned them off in 1bc1cd5
("chore: don't emit sourcemaps in prod", 2024-03-28).

Using recommended config from:

- https://docs.datadoghq.com/real_user_monitoring/guide/upload-javascript-source-maps/?tab=webpackjs

webpack.production.config.js

@@ -51,6 +51,7 @@ module.exports = BUILDS.map(({name, filename, targets, terserOptions}) => {
       ],
     },
     mode: 'production',
+    devtool: false,
     node: false,
     plugins: [
       new AssetsPlugin({
@@ -71,6 +72,10 @@ module.exports = BUILDS.map(({name, filename, targets, terserOptions}) => {
             ')\\b',
         ),
       }),
+      new webpack.SourceMapDevToolPlugin({
+        noSources: false,
+        filename: '[file].map',
+      }),
       function () {
         this.hooks.done.tapAsync(
           'copy-assets-from-dist-to-public-static',