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/TaskDocumentList.vue

@@ -34,6 +34,7 @@
                 <!-- Delete button -->
                 <button
                     v-if="isAdmin"
+                    type="button"
                     class="absolute right-1 top-1 hidden rounded p-0.5 text-neutral-400 transition-colors hover:bg-red-50 hover:text-red-500 group-hover:block"
                     @click.stop="$emit('delete', doc)"
                 >