Fix calendar event description not preserving line breaks (#30329)

Add `white-space: pre-line` to the event description style so that
newlines in the calendar event description are rendered correctly
instead of being collapsed into a single line.

src/panels/calendar/dialog-calendar-event-detail.ts

@@ -271,6 +271,7 @@ class DialogCalendarEventDetail extends LitElement {
           color: var(--secondary-text-color);
           max-width: 300px;
           overflow-wrap: break-word;
+          white-space: pre-line;
         }
       `,
     ];