feat: update DatePicker to handle date selection via onSelect callback in MediaFormModal

src/lib/MediaFormModal.svelte

@@ -109,6 +109,7 @@
           <label class="font-medium text-gray-700 whitespace-nowrap" for="date">日期</label>
           <DatePicker
             value={media.date}
+            onSelect={(date: string) => media.date = date}
             placeholder="选择日期"
           />
         </div>