feat(files-editor): mount auto-language editor + dropdown override

The modal textarea opts in with data-editor-language=auto; the editor
derives the language from the filename extension on each modal open.
A dropdown lets the user override (srccfg / bash / plain). The
existing fetch-based /files/save path is unchanged — files-overlay.js
keeps reading textarea.value, which the editor mirrors.

l4d2web/l4d2web/static/js/files-overlay.js

@@ -281,6 +281,16 @@
     saveBtn: editorDialog.querySelector(".files-editor-save"),
   };
 
+  function setEditorContent(text) {
+    const editor = editorEls.contentBox._codeEditor;
+    if (editor) {
+      editor.setValue(text);
+      editor.setLanguage("auto"); // re-derive from filename
+    } else {
+      editorEls.contentBox.value = text;
+    }
+  }
+
   function setEditorTitle(text) {
     editorEls.title.textContent = text;
   }
@@ -342,7 +352,7 @@
     setEditorTitle(`${folder ? folder + "/" : ""}…new file`);
     editorEls.filename.value = "";
     editorEls.filename.disabled = false;
-    editorEls.contentBox.value = "";
+    setEditorContent("");
     editorEls.contentBox.disabled = false;
     editorEls.renameHint.hidden = true;
     editorEls.textPanel.hidden = false;
@@ -375,14 +385,14 @@
       editor.mode = "text";
       editorEls.textPanel.hidden = false;
       editorEls.binaryPanel.hidden = true;
-      editorEls.contentBox.value = "Loading…";
+      setEditorContent("Loading…");
       editorEls.contentBox.disabled = true;
 
       const r = await fetchJson(
         `${baseUrl}/files/content?path=${encodeURIComponent(path)}`
       );
       if (r.ok && r.body) {
-        editorEls.contentBox.value = r.body.content;
+        setEditorContent(r.body.content);
         editorEls.contentBox.disabled = false;
         updateByteCount();
         updateSaveEnabled();
@@ -407,9 +417,21 @@
     setTimeout(() => editorEls.filename.focus(), 0);
   }
 
+  const languageSelect = document.querySelector(".files-editor-language");
+  if (languageSelect) {
+    languageSelect.addEventListener("change", function () {
+      const editor = editorEls.contentBox._codeEditor;
+      if (editor) editor.setLanguage(languageSelect.value);
+    });
+  }
+
   editorEls.filename.addEventListener("input", () => {
     updateRenameHint();
     updateSaveEnabled();
+    const _editor = editorEls.contentBox._codeEditor;
+    if (_editor && languageSelect && languageSelect.value === "auto") {
+      _editor.setLanguage("auto");
+    }
   });
   editorEls.contentBox.addEventListener("input", () => {
     updateByteCount();

l4d2web/l4d2web/templates/overlay_detail.html

@@ -175,7 +175,16 @@
     <div class="files-editor-text">
       <label class="files-editor-field">
         <span class="files-field-label">Content</span>
-        <textarea class="files-editor-content" rows="14" spellcheck="false"></textarea>
+        <textarea class="files-editor-content" rows="14" spellcheck="false" data-editor-language="auto"></textarea>
+      </label>
+      <label class="files-editor-field files-editor-language-field">
+        <span class="files-field-label">Language</span>
+        <select class="files-editor-language editor-language-select">
+          <option value="auto" selected>Auto (from filename)</option>
+          <option value="srccfg">Source config (.cfg)</option>
+          <option value="bash">Bash (.sh)</option>
+          <option value="plain">Plain text</option>
+        </select>
       </label>
       <div class="files-editor-meta muted">
         <span class="files-editor-byte-count">UTF-8 · 0 bytes</span>