fix(ui5-date-picker): use variable for popover width in compact density mode (#12305)

The DatePicker popover was displaying unnecessary whitespace when the content density was set to "compact". This occurred because the popover width was hardcoded to 20rem in the CSS, while the calendar content inside adapts to different widths based on the content density:
Default mode: Calendar width is 20rem (320px)
Compact mode: Calendar width is 17.75rem (284px)

Fixes #12277.

Author: Copilot

packages/main/src/themes/DatePickerPopover.css

@@ -1,5 +1,5 @@
 .ui5-date-picker-popover {
-	width: 20rem;
+	width: var(--_ui5_calendar_width);
 	.ui5-popover-header {
 		width: 100%;
 	}