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/subscriptions/PreviewDialog.vue

@@ -6,10 +6,10 @@
     @update:model-value="$emit('update:modelValue', $event)"
   >
     <v-card v-if="source">
-      <v-card-title class="d-flex align-center">
-        <v-icon class="mr-2">mdi-eye-outline</v-icon>
-        Preview · {{ source.artist_name }}
-      </v-card-title>
+      <CardHeading
+        icon="mdi-eye-outline"
+        :title="`Preview · ${source.artist_name}`"
+      />
       <v-card-subtitle class="fc-preview__url">{{ source.url }}</v-card-subtitle>
 
       <v-card-text>
@@ -70,6 +70,7 @@
 import { ref, watch } from 'vue'
 import { useSourcesStore } from '../../stores/sources.js'
 import { formatRelative } from '../../utils/date.js'
+import CardHeading from '../common/CardHeading.vue'
 
 const props = defineProps({
   modelValue: { type: Boolean, default: false },