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/artist/ArtistDangerZone.vue

@@ -1,9 +1,6 @@
 <template>
   <v-card class="fc-danger-zone mt-8" variant="outlined">
-    <v-card-title class="d-flex align-center" style="gap: 10px;">
-      <v-icon icon="mdi-alert-octagon" color="error" size="small" />
-      <span>Danger zone</span>
-    </v-card-title>
+    <CardHeading icon="mdi-alert-octagon" icon-color="error" title="Danger zone" />
     <v-card-text>
       <p class="text-body-2 fc-muted mb-4">
         Cascade-delete this artist and every image, source, post, and
@@ -37,6 +34,7 @@ import { useRouter } from 'vue-router'
 
 import DestructiveConfirmModal from '../modal/DestructiveConfirmModal.vue'
 import { useAdminStore } from '../../stores/admin.js'
+import CardHeading from '../common/CardHeading.vue'
 
 const props = defineProps({
   slug: { type: String, required: true },