feat(DateTimePicker): implement custom popper container for improved layout handling

feat(Modal): enhance modal styling with rounded corners and improved overflow handling
style: adjust z-index for datepicker popper to ensure proper layering above modals

src/index.css

@@ -823,6 +823,11 @@
   display: none;
 }
 
+/* Portal-rendered popper must sit above modals (z-50 = 50 in Tailwind) */
+.react-datepicker-popper {
+  z-index: 9999 !important;
+}
+
 /* ── Header (custom rendered) ─────────────────────────────────── */
 .dtp-calendar .react-datepicker__header {
   background: var(--bg-secondary);