feat(editor): widget core — mount, sync, language switch

Mounts on <textarea data-editor-language>, hides the textarea, renders
content in a contenteditable sibling with Prism highlighting via
CodeJar. Mirrors content back to textarea.value on every input so form
POST and existing JS readers keep working unchanged. Exposes
setValue/setLanguage/getValue on textarea._codeEditor for callers.

Language switch uses tear-down-and-remount because CodeJar captures
its highlighter by closure at construction time and has no API to
swap it on a live instance.

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

l4d2web/l4d2web/static/js/editor.js

@@ -0,0 +1,147 @@
+// Code editor widget. Mounts on any <textarea data-editor-language>.
+// The textarea stays in the DOM (display:none) and the widget mirrors
+// content back into it on every input — form submission and JS code
+// that reads textarea.value (e.g. files-overlay.js) keep working.
+//
+// Public per-instance API (attached to the textarea as ._codeEditor):
+//   - setValue(text)
+//   - setLanguage(name)   // "srccfg" | "bash" | "plain" | "auto"
+//   - getValue()
+//   - destroy()
+(function () {
+  "use strict";
+
+  const LANG_BY_EXT = {
+    cfg: "srccfg",
+    sh: "bash",
+    bash: "bash",
+  };
+
+  function resolveAutoLanguage(filename) {
+    if (!filename) return "plain";
+    const m = /\.([a-zA-Z0-9]+)$/.exec(filename);
+    if (!m) return "plain";
+    return LANG_BY_EXT[m[1].toLowerCase()] || "plain";
+  }
+
+  function highlightFor(lang) {
+    return function (editorEl) {
+      if (lang === "plain" || !window.Prism || !window.Prism.languages[lang]) {
+        // Plain mode or grammar missing: leave textContent alone.
+        editorEl.innerHTML = editorEl.textContent
+          .replace(/&/g, "&amp;")
+          .replace(/</g, "&lt;");
+        return;
+      }
+      window.Prism.highlightElement(editorEl);
+    };
+  }
+
+  // For "auto" language: look for a filename input near the textarea
+  // (the files-editor modal). Returns the <input> or null.
+  function findFilenameInput(textarea) {
+    const modal = textarea.closest("dialog, .modal, body");
+    if (!modal) return null;
+    return modal.querySelector(".files-editor-filename");
+  }
+
+  function mount(textarea) {
+    if (textarea._codeEditor) return textarea._codeEditor;
+
+    const requested = textarea.dataset.editorLanguage || "plain";
+    let language =
+      requested === "auto"
+        ? resolveAutoLanguage(findFilenameInput(textarea)?.value)
+        : requested;
+
+    // Build the visible editor.
+    const shell = document.createElement("div");
+    shell.className = "editor-shell";
+    const code = document.createElement("code");
+    code.className = "editor-code language-" + language;
+    code.setAttribute("contenteditable", "true");
+    code.setAttribute("spellcheck", "false");
+    code.textContent = textarea.value;
+    shell.appendChild(code);
+    textarea.parentNode.insertBefore(shell, textarea);
+    textarea.style.display = "none";
+
+    // CodeJar mounts on the contenteditable and re-runs the highlighter
+    // on each input while preserving caret position.
+    const jar = window.CodeJar(code, highlightFor(language), { tab: "  " });
+
+    function attachOnUpdate(jarInstance) {
+      jarInstance.onUpdate(function (value) {
+        // Mirror back to the underlying textarea so form POST and any
+        // .value readers see the current content.
+        textarea.value = value;
+        textarea.dispatchEvent(new Event("input", { bubbles: true }));
+      });
+    }
+
+    attachOnUpdate(jar);
+
+    const instance = {
+      textarea,
+      shell,
+      code,
+      jar,
+      language,
+      setValue: function (text) {
+        instance.jar.updateCode(text);
+        textarea.value = text;
+      },
+      getValue: function () {
+        return instance.jar.toString();
+      },
+      setLanguage: function (name) {
+        const next =
+          name === "auto"
+            ? resolveAutoLanguage(findFilenameInput(textarea)?.value)
+            : name;
+        if (next === instance.language) return;
+        // CodeJar captures its highlight callback by closure at
+        // construction time — there is no API to swap it on a live
+        // instance. Tear down and remount with the new highlighter.
+        // Caret position is lost on switch; acceptable since this is
+        // triggered by the user clicking the language dropdown.
+        const currentText = instance.jar.toString();
+        instance.jar.destroy();
+        instance.language = next;
+        code.className = "editor-code language-" + next;
+        code.textContent = currentText;
+        instance.jar = window.CodeJar(code, highlightFor(next), { tab: "  " });
+        attachOnUpdate(instance.jar);
+      },
+      destroy: function () {
+        instance.jar.destroy();
+        shell.remove();
+        textarea.style.display = "";
+        delete textarea._codeEditor;
+      },
+    };
+
+    textarea._codeEditor = instance;
+    return instance;
+  }
+
+  function mountAll(root) {
+    const scope = root || document;
+    scope.querySelectorAll("textarea[data-editor-language]").forEach(mount);
+  }
+
+  if (document.readyState === "loading") {
+    document.addEventListener("DOMContentLoaded", function () {
+      mountAll(document);
+    });
+  } else {
+    mountAll(document);
+  }
+
+  // Re-export for callers that need to mount editors created later
+  // (the files-editor modal exists in the static DOM but is only
+  // shown after user interaction — initial mount is correct, but
+  // exposing this hook lets future code mount dynamically-inserted
+  // editors if needed).
+  window.l4d2Editor = { mount, mountAll };
+})();