feat(ui): ErrorDetailModal — context panel (task/queue/target/duration/started/retries/worker/celery-id/args) + contrast fix (background-token bg vs surface-variant pale-on-pale) + copyText helper for Copy button

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/components/settings/SystemActivityTab.vue

@@ -62,7 +62,7 @@
               <td>
                 <button
                   type="button" class="fc-err-link"
-                  @click="openError(r.error_type, r.error_message)"
+                  @click="openError(r)"
                   :title="'Click for full error'"
                 >{{ r.error_type }}</button>
               </td>
@@ -145,8 +145,7 @@
 
     <ErrorDetailModal
       v-model="showErrorModal"
-      :title="errorModalTitle"
-      :message="errorModalMessage"
+      :row="errorModalRow"
     />
   </div>
 </template>
@@ -163,12 +162,10 @@ import QueuesTable from './QueuesTable.vue'
 // rollback + traceback content rendered as a cramped browser tooltip
 // you couldn't copy from or scroll within).
 const showErrorModal = ref(false)
-const errorModalTitle = ref('')
-const errorModalMessage = ref('')
+const errorModalRow = ref(null)
 
-function openError(title, message) {
-  errorModalTitle.value = title || 'Error details'
-  errorModalMessage.value = message || ''
+function openError(row) {
+  errorModalRow.value = row
   showErrorModal.value = true
 }