style(workspace): make pin button icon-only matching trash button

frontend/src/features/workspace/WorkspaceView.tsx

@@ -1286,10 +1286,14 @@ export const WorkspaceView: React.FC<WorkspaceViewProps> = ({
                       onClick={() =>
                         persistDocument({ ...selectedDoc, isPinned: !selectedDoc.isPinned })
                       }
-                      className={selectedDoc.isPinned ? 'btn-action text-accent border-accent/40' : 'btn-action'}
+                      className={
+                        selectedDoc.isPinned
+                          ? 'btn-icon text-accent border-accent/40 bg-accent/10'
+                          : 'btn-icon'
+                      }
+                      title={selectedDoc.isPinned ? 'Von Favoriten / Angepinnt entfernen' : 'Oben in Favoriten anpinnen'}
                     >
                       <Pin className="w-3.5 h-3.5" />
-                      <span>{selectedDoc.isPinned ? 'Angepinnt' : 'Anpinnen'}</span>
                     </button>
 
                     <button