feat(06.2-03): frontend — CloudDeleteWarningModal + remove_only path in DocumentView

- api/client.js: deleteDocument gains removeOnly param; deleteDocumentRemoveOnly wrapper added
- DocumentView.vue: confirmDelete inspects response.cloud_delete_failed, shows modal on failure
- DocumentView.vue: inline CloudDeleteWarningModal (C-3 contract) with Remove from app / Cancel
- confirmRemoveOnly() calls DELETE ?remove_only=true and navigates to /

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

frontend/src/api/client.js

@@ -72,8 +72,13 @@ export function getDocument(id) {
   return request(`/api/documents/${id}`)
 }
 
-export function deleteDocument(id) {
-  return request(`/api/documents/${id}`, { method: 'DELETE' })
+export function deleteDocument(id, removeOnly = false) {
+  const url = removeOnly ? `/api/documents/${id}?remove_only=true` : `/api/documents/${id}`
+  return request(url, { method: 'DELETE' })
+}
+
+export function deleteDocumentRemoveOnly(id) {
+  return deleteDocument(id, true)
 }
 
 export function classifyDocument(id, topics = null) {