Enforce no two groups on same cell, allow trainer overlap

- Group-to-group segment overlap is blocked (hard error)
- Trainer-to-group overlap is allowed (trainers supervise areas)
- Checks pending unsaved assignments, not just DB reservations
- Proper Arabic error message explaining the constraint
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/Livewire/Facilities/VisualScheduleBuilder.php

@@ -173,7 +173,28 @@ public function assignGroupToSegments(int $groupId, array $segmentIds): void
             return;
         }
 
-        // 2. Check group isn't already assigned in this slot (in current unsaved assignments)
+        // 2. Check segments aren't already claimed by another group (group-to-group overlap is blocked)
+        //    Trainers CAN overlap with groups (supervision), but two groups cannot share cells.
+        foreach ($this->assignments as $assignment) {
+            $assignType = $assignment['type'] ?? '';
+            if ($assignType === 'trainer') continue; // trainers don't block groups
+
+            $occupiedSegments = $assignment['segment_ids'] ?? [];
+            $overlap = array_intersect($segmentIds, $occupiedSegments);
+
+            if (!empty($overlap)) {
+                $ownerName = $assignment['group_name'] ?? $assignment['title'] ?? __('مجموعة أخرى');
+                $this->conflicts = [[
+                    'category' => 'segment_taken',
+                    'title' => $ownerName,
+                    'message' => __('الخلايا المحددة مشغولة بالفعل بواسطة') . ' "' . $ownerName . '" — ' . __('لا يمكن لمجموعتين مشاركة نفس المساحة'),
+                    'time' => $this->selectedSlotStart . ' - ' . $this->selectedSlotEnd,
+                ]];
+                return;
+            }
+        }
+
+        // 3. Check same group isn't already assigned in this slot
         foreach ($this->assignments as $assignment) {
             if (($assignment['group_id'] ?? null) === $groupId && ($assignment['type'] ?? '') !== 'existing') {
                 $this->addError('assignment', __('هذه المجموعة معيّنة بالفعل في هذا الوقت'));