feat(settings): tidy Maintenance tab into compact tiles + center the views (pass 2)

Goal (operator 2026-06-18): the overview of a Settings tab fits one unscrolled
viewport; expanding a tile to read into it is the only reason to scroll.

- Every Maintenance card converted to the collapsible MaintenanceTile (collapsed
  by default = icon + short title + one-line blurb). Task cards (ML backfill,
  centroids, thumbnails, archive re-extract, missing-file repair, DB maintenance)
  sit in a responsive grid; running tasks auto-expand. Tagging config (suggestion
  thresholds, allowlist, aliases) grouped in one Tagging section as collapsible
  tiles; Backup is its own collapsible tile.
- Three labeled sections mirror the Cleanup tab: Backfills and reprocessing /
  Tagging / Storage.
- Center the whole Settings surface: SettingsView is now a centered, width-capped
  (1140px) column so the tab strip and every panel sit in a tidy centered measure
  (was full-width). CleanupView drops its own left-aligned max-width to fill it.

All card logic unchanged - only the chrome.

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

frontend/src/components/settings/MLThresholdSliders.vue

@@ -1,7 +1,10 @@
 <template>
-  <v-card>
-    <v-card-title>Suggestion thresholds</v-card-title>
-    <v-card-text v-if="store.settings">
+  <MaintenanceTile
+    icon="mdi-tune"
+    title="Suggestion thresholds"
+    blurb="Confidence cutoffs that gate auto-suggested tags + video sampling."
+  >
+    <div v-if="store.settings">
       <v-row v-for="f in fields" :key="f.key">
         <v-col cols="12">
           <v-slider
@@ -63,15 +66,16 @@
           />
         </v-col>
       </v-row>
-    </v-card-text>
-    <v-card-text v-else><v-skeleton-loader type="paragraph" /></v-card-text>
-  </v-card>
+    </div>
+    <div v-else><v-skeleton-loader type="paragraph" /></div>
+  </MaintenanceTile>
 </template>
 
 <script setup>
 import { toast } from '../../utils/toast.js'
 import { reactive, watch } from 'vue'
 import { useMLStore } from '../../stores/ml.js'
+import MaintenanceTile from '../common/MaintenanceTile.vue'
 
 const store = useMLStore()
 // 'artist' (FC-2d-vii-c) and 'copyright' (2026-06-01) retired as