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/ui/ConfirmDeleteDocumentModal.vue

@@ -2,7 +2,7 @@
     <Teleport v-if="modelValue" to="body">
         <Transition name="modal" appear>
             <div class="fixed inset-0 z-[70] flex items-center justify-center">
-                <div class="absolute inset-0 bg-black/30" @click="cancel" />
+                <div class="absolute inset-0 bg-black/30" @click.stop="cancel" />
                 <div class="relative z-10 w-full max-w-md rounded-lg bg-white p-6 shadow-xl">
                     <h3 class="text-lg font-bold text-neutral-900">{{ $t('taskDocuments.confirmDeleteTitle') }}</h3>
                     <p class="mt-3 text-sm text-neutral-600">