fix(ui): pin desktop calendar sidebar

frontend/src/components/AppShell.tsx

@@ -469,6 +469,7 @@ export function AppShell({ children }: AppShellProps) {
             setEventsRefreshTrigger={cal.handleSetEventsRefreshTrigger}
             setSidebarEvents={cal.handleSetSidebarEvents}
             onCalendarChange={cal.handleCalendarChange}
+            fixedDesktop={!isMessageDialogPage}
           />
         </div>
 

frontend/src/components/calendar/CalendarSidebar.tsx

@@ -15,16 +15,22 @@ interface CalendarSidebarProps {
   setEventsRefreshTrigger: (value: number | ((prev: number) => number)) => void;
   setSidebarEvents: (events: CalendarEvent[]) => void;
   onCalendarChange: (calendarId: number | null) => void;
+  fixedDesktop?: boolean;
 }
 
 /**
  * Обертка для календаря в правом сайдбаре (десктоп)
  */
 export const CalendarSidebar = memo(function CalendarSidebar(props: CalendarSidebarProps) {
+  const { fixedDesktop = false, ...calendarProps } = props;
   return (
     <aside className="hidden w-72 flex-shrink-0 space-y-4 lg:block">
-      <div className="sticky top-8 space-y-4 lg:pb-2 max-h-[calc(100vh-7.5rem)] overflow-y-auto">
-        <CalendarCard {...props} />
+      <div
+        className={`space-y-4 lg:max-h-[calc(100vh-7.5rem)] lg:overflow-y-auto lg:pb-2 ${
+          fixedDesktop ? "lg:fixed lg:top-[5.5rem] lg:w-72" : "lg:sticky lg:top-8"
+        }`}
+      >
+        <CalendarCard {...calendarProps} />
       </div>
     </aside>
   );