refactor(scheduling): split multi-day blockers and normalize time strings

Split multi-day time blockers into per-day segments for subtractTimeSlots. Add normalizeTime helper to strip seconds from HH:MM:SS for consistent comparison. Subtract blockers for all users (not just non-admin) to prevent 409 errors on reserve. Move late-night lock logic to separate block.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

backend/handlers/scheduling/default-hours.go

@@ -492,18 +492,33 @@ func GetAvailableHours(w http.ResponseWriter, r *http.Request) {
 	blockerMap := make(map[string][]TimeSlot)
 
 	for _, blocker := range blockers {
+		blockStart := blocker.StartTime
+		blockEnd := blockStart.Add(time.Duration(blocker.DurationMinutes) * time.Minute)
 
-		dateStr := blocker.StartTime.Format("2006-01-02")
+		// Split multi-day blockers into per-day segments so subtractTimeSlots
+		// only compares times within the same calendar day. Each segment's end
+		// time uses "24:00" for day boundaries (midnight of the next day) since
+		// "00:00" as an end time would incorrectly appear before all slot times.
+		cur := blockStart
+		for cur.Before(blockEnd) {
+			dayEnd := time.Date(cur.Year(), cur.Month(), cur.Day(), 0, 0, 0, 0, cur.Location()).AddDate(0, 0, 1)
+			segEnd := blockEnd
+			if segEnd.After(dayEnd) {
+				segEnd = dayEnd
+			}
 
-		endTime := blocker.StartTime.Add(time.Duration(blocker.DurationMinutes) * time.Minute)
-
-		blockerMap[dateStr] = append(blockerMap[dateStr], TimeSlot{
-
-			StartTime: blocker.StartTime.Format("15:04"),
-
-			EndTime: endTime.Format("15:04"),
-		})
+			dateStr := cur.Format("2006-01-02")
+			endStr := segEnd.Format("15:04")
+			if segEnd.Equal(dayEnd) {
+				endStr = "24:00"
+			}
+			blockerMap[dateStr] = append(blockerMap[dateStr], TimeSlot{
+				StartTime: cur.Format("15:04"),
+				EndTime:   endStr,
+			})
 
+			cur = dayEnd
+		}
 	}
 
 	// Check if user is admin
@@ -575,21 +590,28 @@ func GetAvailableHours(w http.ResponseWriter, r *http.Request) {
 
 		day.IsOpen = isOpen
 		if isOpen {
+			// Normalize to HH:MM to match blocker and booking time formats
+			baseStart = normalizeTime(baseStart)
+			baseEnd = normalizeTime(baseEnd)
 			slots := []TimeSlot{{StartTime: baseStart, EndTime: baseEnd}}
 			if booked, ok := bookings[day.Date]; ok {
 				slots = subtractTimeSlots(slots, booked)
 			}
 			day.Slots = slots
-			// Handle time blockers
-			if !isAdmin {
-				// Regular users: subtract blockers from available slots
-				if dayBlockers, ok := blockerMap[day.Date]; ok {
-					day.Slots = subtractTimeSlots(day.Slots, dayBlockers)
+			// Subtract time blockers from available slots for ALL users
+			// (prevents showing blocked slots that would fail on reserve)
+			if dayBlockers, ok := blockerMap[day.Date]; ok {
+				day.Slots = subtractTimeSlots(day.Slots, dayBlockers)
+				// Store blockers separately for admin warning display
+				if isAdmin {
+					day.Blockers = dayBlockers
 				}
+			}
 
-				// Late night lock: after 22:00, block next morning 00:00-11:00 for non-admin users
+			// Late night lock: after 22:00, block next morning 00:00-11:00 for non-admin users
+			if !isAdmin {
 				now := time.Now()
-				if !isAdmin && now.Hour() >= 22 {
+				if now.Hour() >= 22 {
 					// Check if this is tomorrow's date
 					tomorrow := now.AddDate(0, 0, 1)
 					tomorrowStr := tomorrow.Format("2006-01-02")
@@ -602,11 +624,6 @@ func GetAvailableHours(w http.ResponseWriter, r *http.Request) {
 						day.Slots = subtractTimeSlots(day.Slots, []TimeSlot{lateNightBlock})
 					}
 				}
-			} else {
-				// Admins: keep blockers visible for warning display
-				if dayBlockers, ok := blockerMap[day.Date]; ok {
-					day.Blockers = dayBlockers
-				}
 			}
 		} else {
 			day.Slots = []TimeSlot{}
@@ -619,6 +636,20 @@ func GetAvailableHours(w http.ResponseWriter, r *http.Request) {
 	json.NewEncoder(w).Encode(results)
 }
 
+// normalizeTime strips seconds from HH:MM:SS to HH:MM for consistent string
+// comparison with blocker and booking time formats in subtractTimeSlots.
+func normalizeTime(t string) string {
+	// Strip trailing :SS (seconds) from HH:MM:SS format while leaving
+	// bare HH:MM untouched and handling single-digit hours (e.g. 9:00:00).
+	if len(t) > 5 && t[len(t)-3] == ':' {
+		prefix := t[:len(t)-3]
+		if strings.Contains(prefix, ":") {
+			return prefix
+		}
+	}
+	return t
+}
+
 // subtractTimeSlots removes gaps from available slots
 // Returns the remaining available time slots after removing the gaps
 func subtractTimeSlots(available []TimeSlot, gaps []TimeSlot) []TimeSlot {