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/prompts/PromptListItem.tsx

@@ -36,11 +36,11 @@ const PromptListItem: React.FC<PromptListItemProps> = ({
         </div>
 
         <div className="flex-1 min-w-0">
-          <h3 className="font-medium text-gray-900 dark:text-gray-100 mb-1">
+          <h3 className="font-medium text-foreground mb-1">
             {prompt.name}
           </h3>
           {prompt.description && (
-            <p className="text-sm text-gray-500 dark:text-gray-400 truncate">
+            <p className="text-sm text-muted-foreground truncate">
               {prompt.description}
             </p>
           )}

src/components/prompts/PromptPanel.tsx

@@ -108,21 +108,21 @@ const PromptPanel = React.forwardRef<PromptPanelHandle, PromptPanelProps>(
 
         <div className="flex-1 overflow-y-auto pb-16">
           {loading ? (
-            <div className="text-center py-12 text-gray-500 dark:text-gray-400">
+            <div className="text-center py-12 text-muted-foreground">
               {t("prompts.loading")}
             </div>
           ) : promptEntries.length === 0 ? (
             <div className="text-center py-12">
-              <div className="w-16 h-16 mx-auto mb-4 bg-gray-100 dark:bg-gray-800 rounded-full flex items-center justify-center">
+              <div className="w-16 h-16 mx-auto mb-4 bg-muted rounded-full flex items-center justify-center">
                 <FileText
                   size={24}
-                  className="text-gray-400 dark:text-gray-500"
+                  className="text-muted-foreground"
                 />
               </div>
-              <h3 className="text-lg font-medium text-gray-900 dark:text-gray-100 mb-2">
+              <h3 className="text-lg font-medium text-foreground mb-2">
                 {t("prompts.empty")}
               </h3>
-              <p className="text-gray-500 dark:text-gray-400 text-sm">
+              <p className="text-muted-foreground text-sm">
                 {t("prompts.emptyDescription")}
               </p>
             </div>