feat(ui): translation-forward post text + Settings card (#143 step 5)

PostCard + modal ProvenancePanel show the English title/description by default when
a translation exists, with a per-card "show original (<lang>)" toggle — translated
bodies render as plain text, originals keep their sanitized HTML. New
TranslationCard in Settings → Ingestion & filters: enable switch, Interpreter base
URL (generic placeholder, no default host), target language, a reachability
indicator + untranslated-posts count + "Translate now".

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CDgx8bQS5YrGRK76v8HUnM

frontend/src/components/settings/MaintenancePanel.vue

@@ -13,6 +13,7 @@
       </p>
       <div class="fc-tile-stack">
         <ImportFiltersForm />
+        <TranslationCard />
       </div>
     </section>
 
@@ -69,6 +70,7 @@
 import { onMounted, onUnmounted } from 'vue'
 
 import ImportFiltersForm from './ImportFiltersForm.vue'
+import TranslationCard from './TranslationCard.vue'
 import MLBackfillCard from './MLBackfillCard.vue'
 import ThumbnailBackfillCard from './ThumbnailBackfillCard.vue'
 import ArchiveReextractCard from './ArchiveReextractCard.vue'