Inject jQuery for legacy plugins

Legacy jQuery plugins need to attach themselves to the same jQuery
instance that application code imports from ESM. Wrap enjoyhint and
tablesorter with that instance while they load so precompiled assets do
not route their UMD wrappers through a separate CommonJS jQuery copy.

frontend/esbuild/plugins.ts

@@ -29,6 +29,8 @@
  */
 
 import type { Plugin } from 'esbuild';
+import * as fs from 'fs';
+import * as path from 'node:path';
 
 const customConfigPlugin:Plugin = {
   name: 'custom-config',
@@ -36,7 +38,68 @@ const customConfigPlugin:Plugin = {
     if (options.chunkNames === '[name]-[hash]') { // named chunks
       options.chunkNames = '[dir]/[name]-[hash]';
     }
-  }
+  },
+};
+
+const jqueryInjectionPlugin:Plugin = {
+  name: 'jquery-injection',
+  setup(build) {
+    // Intercept legacy jQuery plugins that need the ESM jQuery instance.
+    build.onResolve({ filter: /^(core-vendor\/enjoyhint|tablesorter)$/ }, (args) => {
+      return {
+        path: args.path,
+        namespace: 'jquery-wrapper',
+      };
+    });
+
+    // Provide the wrapper content
+    build.onLoad({ filter: /.*/, namespace: 'jquery-wrapper' }, async (args) => {
+      const workingDir = build.initialOptions.absWorkingDir ?? process.cwd();
+      const modulePath = args.path === 'tablesorter'
+        ? path.join(workingDir, 'node_modules', 'tablesorter', 'dist', 'js', 'jquery.tablesorter.combined.js')
+        : path.join(workingDir, 'src', 'vendor', 'enjoyhint.js');
+      const contents = await fs.promises.readFile(modulePath, 'utf8');
+
+      // Wrap with jQuery import
+      const wrappedCode = `
+import jQuery from 'jquery';
+import 'jquery-migrate';
+
+const previousJQuery = window.jQuery;
+const previousDollar = window.$;
+const hadPreviousJQuery = 'jQuery' in window;
+const hadPreviousDollar = '$' in window;
+
+// Legacy jQuery plugins expect global jQuery while they load.
+window.jQuery = jQuery;
+window.$ = jQuery;
+
+const define = undefined;
+const module = undefined;
+const exports = undefined;
+
+${contents}
+
+if (hadPreviousJQuery) {
+  window.jQuery = previousJQuery;
+} else {
+  delete window.jQuery;
 }
 
-export default [customConfigPlugin];
+if (hadPreviousDollar) {
+  window.$ = previousDollar;
+} else {
+  delete window.$;
+}
+`;
+
+      return {
+        contents: wrappedCode,
+        loader: 'js',
+        resolveDir: path.join(workingDir, 'src'),
+      };
+    });
+  },
+};
+
+export default [customConfigPlugin, jqueryInjectionPlugin];