diff --git a/l4d2web/scripts/editor-src/editor-entry.js b/l4d2web/scripts/editor-src/editor-entry.js
index 4be7802..444ecdb 100644
--- a/l4d2web/scripts/editor-src/editor-entry.js
+++ b/l4d2web/scripts/editor-src/editor-entry.js
@@ -1,2 +1,84 @@
-// Façade entry point. Populated in Task 4.
-window.__editor = { mount: () => { throw new Error('editor bundle not yet implemented'); } };
+import { EditorState, Compartment } from "@codemirror/state";
+import { EditorView, keymap, lineNumbers, highlightActiveLine } from "@codemirror/view";
+import { defaultKeymap, history, historyKeymap, indentWithTab } from "@codemirror/commands";
+import { StreamLanguage, indentOnInput, bracketMatching, defaultHighlightStyle, syntaxHighlighting } from "@codemirror/language";
+import { closeBrackets, closeBracketsKeymap, completionKeymap } from "@codemirror/autocomplete";
+import { shell as shellLegacy } from "@codemirror/legacy-modes/mode/shell";
+
+import { srccfgLanguage } from "./srccfg-mode.js";
+import { editorLightTheme, editorDarkTheme, editorHighlighting } from "./themes.js";
+import { autocompleteExtension } from "./autocomplete.js";
+
+const bashLanguage = StreamLanguage.define(shellLegacy);
+
+function pickLanguage(name) {
+  if (name === "srccfg") return srccfgLanguage;
+  if (name === "bash")   return bashLanguage;
+  return null;  // "plain" / unknown → no language extension
+}
+
+function pickThemeForMatchMedia(mm) {
+  return mm.matches ? editorDarkTheme : editorLightTheme;
+}
+
+function mount(textarea, { language = "plain", vocab = null } = {}) {
+  const langCompartment = new Compartment();
+  const themeCompartment = new Compartment();
+  const autocompleteCompartment = new Compartment();
+
+  const lang = pickLanguage(language);
+  const mm = window.matchMedia("(prefers-color-scheme: dark)");
+
+  const extensions = [
+    history(),
+    lineNumbers(),
+    highlightActiveLine(),
+    bracketMatching(),
+    closeBrackets(),
+    indentOnInput(),
+    syntaxHighlighting(defaultHighlightStyle, { fallback: true }),
+    editorHighlighting,
+    themeCompartment.of(pickThemeForMatchMedia(mm)),
+    langCompartment.of(lang ? [lang] : []),
+    autocompleteCompartment.of(vocab ? [autocompleteExtension(vocab)] : []),
+    keymap.of([
+      ...closeBracketsKeymap,
+      ...defaultKeymap,
+      ...historyKeymap,
+      ...completionKeymap,
+      indentWithTab,
+    ]),
+  ];
+
+  const state = EditorState.create({ doc: textarea.value, extensions });
+  const view = new EditorView({ state, parent: textarea.parentElement });
+
+  // Insert the editor right before the textarea, then hide the textarea.
+  textarea.parentElement.insertBefore(view.dom, textarea);
+  textarea.style.display = "none";
+
+  // OS-level theme swap
+  const onThemeChange = () => view.dispatch({
+    effects: themeCompartment.reconfigure(pickThemeForMatchMedia(mm)),
+  });
+  mm.addEventListener("change", onThemeChange);
+
+  const controller = {
+    getValue: () => view.state.doc.toString(),
+    setContent: (text) => {
+      view.dispatch({ changes: { from: 0, to: view.state.doc.length, insert: text } });
+    },
+    setLanguage: (name) => {
+      const next = pickLanguage(name);
+      view.dispatch({ effects: langCompartment.reconfigure(next ? [next] : []) });
+    },
+    destroy: () => {
+      mm.removeEventListener("change", onThemeChange);
+      view.destroy();
+      textarea.style.display = "";
+    },
+  };
+  return controller;
+}
+
+window.__editor = { mount };