feat(settings): tidy Cleanup tab into sectioned compact tiles (pass 1)

The Cleanup + Maintenance sections had ~17 full-width stacked cards with long
descriptions — a hunt to scan. Operator wants compact, sectioned, scannable tiles
(2026-06-18: keep both tabs, group inside, compact tiles in a grid).

New common/MaintenanceTile.vue: a compact expandable tile (icon + short title +
one-line blurb collapsed; click the header to expand the full controls/preview/
result inline; keyboard-accessible button + focus ring;  tints the
icon,  keeps a running task expanded).

Cleanup tab (this pass) restructured into 3 sections — Import-filter audits
(Min dimensions, Transparency, Single-color) / Duplicates & posts (Bare posts,
Duplicate posts, Deduplicate videos, Gated-post previews) / Tags (Unused, Legacy,
Reset content tagging, Standardize casing) — each a responsive grid of tiles.
PostMaintenanceCard split into 2 tiles, TagMaintenanceCard into 4. Moved
VideoDedupCard + GatedPurgeCard from the Maintenance tab here (both are
destructive content cleanup). All card logic unchanged — only the chrome.

Maintenance tab tiling is pass 2 (TODO noted in MaintenancePanel).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/components/settings/GatedPurgeCard.vue

@@ -4,9 +4,13 @@
        matches by content hash, so real content downloaded when access existed is
        provably spared. Preview first, then apply (destructive: deletes the
        matched blurred-preview files). -->
-  <v-card>
-    <v-card-title>Clean up gated-post previews</v-card-title>
-    <v-card-text>
+  <MaintenanceTile
+    icon="mdi-image-off"
+    title="Gated-post previews"
+    blurb="Delete blurred locked-preview images from tier-gated posts."
+    destructive
+    :open="applying || previewing"
+  >
       <p class="text-body-2 mb-3">
         Removes the blurred locked-preview images that were grabbed from
         tier-gated Patreon posts before they were filtered out. It re-walks every
@@ -74,7 +78,6 @@
       </v-alert>
 
       <QueueStatusBar queue="maintenance_long" queue-label="Maintenance" />
-    </v-card-text>
 
     <v-dialog v-model="confirmOpen" max-width="460">
       <v-card>
@@ -92,13 +95,14 @@
         </v-card-actions>
       </v-card>
     </v-dialog>
-  </v-card>
+  </MaintenanceTile>
 </template>
 
 <script setup>
 import { computed, ref } from 'vue'
 
 import { useMaintenanceTask } from '../../composables/useMaintenanceTask.js'
+import MaintenanceTile from '../common/MaintenanceTile.vue'
 import QueueStatusBar from './QueueStatusBar.vue'
 
 const confirmOpen = ref(false)