Disable dates with no availability on booking page

- Fetch availability for all dates on page load
- Track which dates have available slots in state
- Disable date buttons that have no availability
- Add visual styling for disabled dates (reduced opacity, not-allowed cursor)
- Prevent clicking on dates with no availability
- Improves UX by showing which dates are bookable at a glance

frontend/app/booking/page.tsx

@@ -92,6 +92,12 @@ const pageStyles: Record<string, React.CSSProperties> = {
     background: "rgba(167, 139, 250, 0.15)",
     border: "1px solid #a78bfa",
   },
+  dateButtonDisabled: {
+    opacity: 0.4,
+    cursor: "not-allowed",
+    background: "rgba(255, 255, 255, 0.01)",
+    border: "1px solid rgba(255, 255, 255, 0.04)",
+  },
   dateWeekday: {
     color: "#fff",
     fontWeight: 500,
@@ -224,6 +230,8 @@ export default function BookingPage() {
   const [isBooking, setIsBooking] = useState(false);
   const [error, setError] = useState<string | null>(null);
   const [successMessage, setSuccessMessage] = useState<string | null>(null);
+  const [datesWithAvailability, setDatesWithAvailability] = useState<Set<string>>(new Set());
+  const [isLoadingAvailability, setIsLoadingAvailability] = useState(true);
 
   const dates = getDateRange(minAdvanceDays, maxAdvanceDays);
 
@@ -245,6 +253,36 @@ export default function BookingPage() {
     }
   }, []);
 
+  // Fetch availability for all dates on mount
+  useEffect(() => {
+    if (!user || !isAuthorized) return;
+
+    const fetchAllAvailability = async () => {
+      setIsLoadingAvailability(true);
+      const availabilitySet = new Set<string>();
+      
+      // Fetch availability for all dates in parallel
+      const promises = dates.map(async (date) => {
+        try {
+          const dateStr = formatDate(date);
+          const data = await api.get<AvailableSlotsResponse>(`/api/booking/slots?date=${dateStr}`);
+          if (data.slots.length > 0) {
+            availabilitySet.add(dateStr);
+          }
+        } catch (err) {
+          // Silently fail for individual dates - they'll just be marked as unavailable
+          console.error(`Failed to fetch availability for ${formatDate(date)}:`, err);
+        }
+      });
+      
+      await Promise.all(promises);
+      setDatesWithAvailability(availabilitySet);
+      setIsLoadingAvailability(false);
+    };
+
+    fetchAllAvailability();
+  }, [user, isAuthorized, dates]);
+
   useEffect(() => {
     if (selectedDate && user && isAuthorized) {
       fetchSlots(selectedDate);
@@ -252,8 +290,12 @@ export default function BookingPage() {
   }, [selectedDate, user, isAuthorized, fetchSlots]);
 
   const handleDateSelect = (date: Date) => {
-    setSelectedDate(date);
-    setSuccessMessage(null);
+    const dateStr = formatDate(date);
+    // Only allow selection if date has availability
+    if (datesWithAvailability.has(dateStr)) {
+      setSelectedDate(date);
+      setSuccessMessage(null);
+    }
   };
 
   const handleSlotSelect = (slot: BookableSlot) => {
@@ -329,14 +371,20 @@ export default function BookingPage() {
           <h2 style={styles.sectionTitle}>Select a Date</h2>
           <div style={styles.dateGrid}>
             {dates.map((date) => {
-              const isSelected = selectedDate && formatDate(selectedDate) === formatDate(date);
+              const dateStr = formatDate(date);
+              const isSelected = selectedDate && formatDate(selectedDate) === dateStr;
+              const hasAvailability = datesWithAvailability.has(dateStr);
+              const isDisabled = !hasAvailability || isLoadingAvailability;
+              
               return (
                 <button
-                  key={formatDate(date)}
+                  key={dateStr}
                   onClick={() => handleDateSelect(date)}
+                  disabled={isDisabled}
                   style={{
                     ...styles.dateButton,
                     ...(isSelected ? styles.dateButtonSelected : {}),
+                    ...(isDisabled ? styles.dateButtonDisabled : {}),
                   }}
                 >
                   <div style={styles.dateWeekday}>