fix(ui): improve text visibility in dark mode

Replace hardcoded gray color classes with semantic color classes
to fix poor text contrast in dark mode:

- MCP panel: server names, descriptions, tags, app labels
- Prompt panel: prompt names, descriptions, empty states
- Usage footer: timestamps, refresh buttons
- Update badge: close button icon
- API key input: disabled state text
- Env warning banner: source info text

Changes:
- `text-gray-400 dark:text-gray-500` → `text-muted-foreground`
  (fixes reversed dark mode logic)
- `text-gray-500 dark:text-gray-400` → `text-muted-foreground`
- `bg-gray-100 dark:bg-gray-800` → `bg-muted`

src/components/UsageFooter.tsx

@@ -114,7 +114,7 @@ const UsageFooter: React.FC<UsageFooterProps> = ({
         {/* 第一行：更新时间和刷新按钮 */}
         <div className="flex items-center gap-2 justify-end">
           {/* 上次查询时间 */}
-          <span className="text-[10px] text-gray-400 dark:text-gray-500 flex items-center gap-1">
+          <span className="text-[10px] text-muted-foreground/70 flex items-center gap-1">
             <Clock size={10} />
             {lastQueriedAt
               ? formatRelativeTime(lastQueriedAt, now, t)
@@ -128,7 +128,7 @@ const UsageFooter: React.FC<UsageFooterProps> = ({
               refetch();
             }}
             disabled={loading}
-            className="p-1 rounded hover:bg-gray-100 dark:hover:bg-gray-800 transition-colors disabled:opacity-50 flex-shrink-0 text-gray-400 dark:text-gray-500"
+            className="p-1 rounded hover:bg-muted transition-colors disabled:opacity-50 flex-shrink-0 text-muted-foreground"
             title={t("usage.refreshUsage")}
           >
             <RefreshCw size={12} className={loading ? "animate-spin" : ""} />
@@ -191,7 +191,7 @@ const UsageFooter: React.FC<UsageFooterProps> = ({
         <div className="flex items-center gap-2">
           {/* 自动查询时间提示 */}
           {lastQueriedAt && (
-            <span className="text-[10px] text-gray-400 dark:text-gray-500 flex items-center gap-1">
+            <span className="text-[10px] text-muted-foreground/70 flex items-center gap-1">
               <Clock size={10} />
               {formatRelativeTime(lastQueriedAt, now, t)}
             </span>