fix(slot-definitions): make valid_from field required in generate modal

- Set default value to today's date
- Add validation before form submission
- Update UI label to show required field with asterisk
- Fixes validation error: 'valid_from' required field

src/app/[locale]/admin/clubs/[club_id]/slot-definitions/GenerateSlotDefinitionsModal.tsx

@@ -33,7 +33,11 @@ export default function GenerateSlotDefinitionsModal({
   // Form state
   const [preset, setPreset] = useState<SlotDefinitionPreset>('workday_standard');
   const [selectedCourtIds, setSelectedCourtIds] = useState<number[]>([]);
-  const [validFrom, setValidFrom] = useState('');
+  const [validFrom, setValidFrom] = useState(() => {
+    // Default to today's date in YYYY-MM-DD format
+    const today = new Date();
+    return today.toISOString().split('T')[0];
+  });
   const [validTo, setValidTo] = useState('');
   const [showAdvanced, setShowAdvanced] = useState(false);
 
@@ -109,6 +113,11 @@ export default function GenerateSlotDefinitionsModal({
       return;
     }
 
+    if (!validFrom) {
+      setError('Please select a valid from date');
+      return;
+    }
+
     setLoading(true);
     setError(null);
 
@@ -116,9 +125,9 @@ export default function GenerateSlotDefinitionsModal({
       preset,
       court_ids: selectedCourtIds,
       pattern_overrides: buildPatternOverrides(),
+      valid_from: validFrom,
     };
 
-    if (validFrom) request.valid_from = validFrom;
     if (validTo) request.valid_to = validTo;
 
     const result = await generateSlotDefinitions(clubId, request);
@@ -223,7 +232,7 @@ export default function GenerateSlotDefinitionsModal({
           <div className="grid grid-cols-2 gap-4">
             <div>
               <label className="block text-sm font-semibold text-slate-700 mb-2">
-                Valid From (Optional)
+                Valid From <span className="text-red-600">*</span>
               </label>
               <input
                 type="date"