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/BoardPicker.tsx

@@ -91,13 +91,14 @@ export default function BoardPicker({
     }
   };
 
+  const empty = !loading && !selected;
   const label = loading
     ? "加载板块…"
     : selected
       ? selected.name
       : boards.length === 0
         ? "暂无可用板块"
-        : "选择板块";
+        : "请选择板块";
 
   const colorIdx = selected
     ? boardColorIndex(selected, selectedIdx)
@@ -108,19 +109,28 @@ export default function BoardPicker({
       <button
         id={id}
         type="button"
-        className="j13-board-trigger"
+        className={`j13-board-trigger${empty ? " is-empty" : ""}`}
         disabled={disabled || loading || boards.length === 0}
         aria-haspopup="listbox"
         aria-expanded={open}
         aria-controls={listId}
+        aria-invalid={empty || undefined}
         onClick={() => setOpen((v) => !v)}
         onKeyDown={onTriggerKeyDown}
       >
-        <span className={`j13-board-trigger-ico av-${colorIdx}`} aria-hidden>
+        <span
+          className={`j13-board-trigger-ico${selected ? ` av-${colorIdx}` : " is-placeholder"}`}
+          aria-hidden
+        >
           {(selected?.name || "板").slice(0, 1)}
         </span>
-        <span className="flex-1 min-w-0 truncate font-semibold text-[14px]">
-          {label}
+        <span className="j13-board-trigger-copy">
+          <span className="j13-board-trigger-name">{label}</span>
+          {empty ? (
+            <span className="j13-board-trigger-hint">发布前必须选择</span>
+          ) : loading ? (
+            <span className="j13-board-trigger-hint">正在加载</span>
+          ) : null}
         </span>
         <ChevronDown
           size={16}