refactor: complete border unification across all components

- Add border styles to JsonEditor (CodeMirror) with theme-responsive colors
- Update all dialog header/footer dividers to use border-border-default
- Replace remaining border-border instances in settings components
- Ensure all borders (including separators and container borders) use unified design system
- All borders now consistently use CSS variables and respond to light/dark themes

src/components/settings/LanguageSettings.tsx

@@ -18,7 +18,7 @@ export function LanguageSettings({ value, onChange }: LanguageSettingsProps) {
           {t("settings.languageHint")}
         </p>
       </header>
-      <div className="inline-flex gap-1 rounded-md border border-border bg-background p-1">
+      <div className="inline-flex gap-1 rounded-md border border-border-default bg-background p-1">
         <LanguageButton active={value === "zh"} onClick={() => onChange("zh")}>
           {t("settings.languageOptionChinese")}
         </LanguageButton>