refactor(ui): CardHeading primitive for icon+title card/dialog headings (DRY pattern sweep)

The icon+title v-card-title heading (d-flex align-center + gap + <v-icon size=small> +
<span>) was hand-rolled identically in 13 cards/dialogs (15 heading instances).
Consolidate to <CardHeading icon title> (components/common) with an iconColor
prop (error headings) and a default slot for trailing content (spacer+actions,
inline status chip). Adopted everywhere the pattern appears — all-or-nothing per
the hardened DRY process.

Over-DRY guard: plain text-only <v-card-title> one-liners are NOT this pattern
and stay; DownloadDetailModal leads with a status CHIP (not an icon), a different
concept, left alone. §8b: the only remaining d-flex align-center v-card-title is
that intentional variant. Catalog updated.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/components/settings/SystemActivitySummary.vue

@@ -1,8 +1,6 @@
 <template>
   <v-card class="fc-activity-summary">
-    <v-card-title class="d-flex align-center" style="gap: 10px;">
-      <v-icon icon="mdi-pulse" size="small" />
-      <span>System activity (last min)</span>
+    <CardHeading icon="mdi-pulse" title="System activity (last min)">
       <v-spacer />
       <v-btn
         variant="text" size="small" rounded="pill"
@@ -11,7 +9,7 @@
         View activity
         <v-icon end size="small">mdi-arrow-right</v-icon>
       </v-btn>
-    </v-card-title>
+    </CardHeading>
 
     <v-card-text>
       <v-alert
@@ -36,6 +34,7 @@ import { onMounted, onUnmounted } from 'vue'
 
 import { useSystemActivityStore } from '../../stores/systemActivity.js'
 import QueuesTable from './QueuesTable.vue'
+import CardHeading from '../common/CardHeading.vue'
 
 defineEmits(['open-activity'])