feat: 外观支持自定义 CSS/JS，并升级发帖工作台与积分存量看板

后台可注入全站 CSS/JS（SSR 生效，CSS 可热换）；发帖/编辑改为锁一屏工作室；经济看板增加存量健康度；主题令牌与弹层误关一并收紧。

Co-authored-by: Cursor <cursoragent@cursor.com>

frontend/components/MarkdownEditor.tsx

@@ -28,7 +28,6 @@ import {
   Pencil,
   Maximize2,
   Minimize2,
-  Columns2,
   Lock,
   ChevronDown,
   Upload,
@@ -61,7 +60,7 @@ type MarkdownEditorProps = {
   allowReplyHide?: boolean;
 };
 
-type EditorMode = "split" | "write" | "preview";
+type EditorMode = "write" | "preview";
 
 type WrapOpts = {
   before: string;
@@ -126,7 +125,7 @@ export default function MarkdownEditor({
   const taRef = useRef<HTMLTextAreaElement>(null);
   const fileRef = useRef<HTMLInputElement>(null);
   const toolbarRef = useRef<HTMLDivElement>(null);
-  const [mode, setMode] = useState<EditorMode>("split");
+  const [mode, setMode] = useState<EditorMode>("write");
   const deferredValue = useDeferredValue(value);
   const [uploading, setUploading] = useState(false);
   const [fullscreen, setFullscreen] = useState(false);
@@ -940,15 +939,6 @@ export default function MarkdownEditor({
             >
               <Pencil size={12} /> 编写
             </button>
-            <button
-              type="button"
-              role="tab"
-              aria-selected={mode === "split"}
-              className={mode === "split" ? "is-on" : ""}
-              onClick={() => setMode("split")}
-            >
-              <Columns2 size={12} /> 双栏
-            </button>
             <button
               type="button"
               role="tab"