Fix React CSS property conflict: use border instead of borderColor

- Replaced borderColor with full border property in dateButtonSelected
- Replaced borderColor with full border property in slotButtonSelected
- Prevents React warning about mixing shorthand and non-shorthand properties
- Fixes console error when clicking elements on booking page

frontend/app/booking/page.tsx

@@ -90,7 +90,7 @@ const pageStyles: Record<string, React.CSSProperties> = {
   },
   dateButtonSelected: {
     background: "rgba(167, 139, 250, 0.15)",
-    borderColor: "#a78bfa",
+    border: "1px solid #a78bfa",
   },
   dateWeekday: {
     color: "#fff",
@@ -120,7 +120,7 @@ const pageStyles: Record<string, React.CSSProperties> = {
   },
   slotButtonSelected: {
     background: "rgba(167, 139, 250, 0.15)",
-    borderColor: "#a78bfa",
+    border: "1px solid #a78bfa",
   },
   emptyState: {
     fontFamily: "'DM Sans', system-ui, sans-serif",