fix(shortcuts-modal): use theme-aware utilities instead of btn-outline

GROWI's custom btn-outline-secondary mixin applies a non-transparent
background in dark mode. Switch to Bootstrap utility classes (border,
text-secondary) that use CSS variables for automatic theme adaptation.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

apps/app/src/client/components/ShortcutsModal/ShortcutsModal.tsx

@@ -520,7 +520,7 @@ const ShortcutsModalSubstance = (): React.JSX.Element => {
             href="https://docs.growi.org"
             target="_blank"
             rel="noreferrer"
-            className="btn btn-outline-secondary d-inline-flex align-items-center gap-2"
+            className="d-inline-flex align-items-center gap-2 px-3 py-2 border rounded text-secondary text-decoration-none"
           >
             <span className="material-symbols-outlined fs-5">help</span>
             {t('modal_shortcuts.Other Shortcuts')}