fix : prevent document delete button from submitting the TaskModal form

The delete button in TaskDocumentList lacked type="button", causing it to
act as a submit button inside the form, which triggered handleSubmit and
closed the modal before the confirmation dialog could appear. Also added
guards to prevent closing TaskModal while a sub-modal is open.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/components/task/TaskModal.vue

@@ -289,6 +289,7 @@ const isOpen = computed({
 })
 
 function close() {
+    if (confirmDeleteDocOpen.value || confirmDeleteOpen.value) return
     isOpen.value = false
 }
 
@@ -390,6 +391,8 @@ function populateForm(task: Task | null) {
 
 watch(() => props.modelValue, async (open) => {
     if (open) {
+        confirmDeleteDocOpen.value = false
+        documentToDelete.value = null
         populateForm(props.task)
         try {
             clientTickets.value = await clientTicketService.getAll({ project: props.projectId })