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/views/SettingsView.vue

@@ -1,5 +1,9 @@
 <template>
-  <v-container fluid class="pt-2 pb-6">
+  <!-- Centered, width-capped column so the tab strip + every panel sit in a
+       tidy centered measure rather than stretching full-width (operator
+       2026-06-18). v-container is margin-auto by default; the cap keeps long
+       maintenance/import panels readable. -->
+  <v-container class="pt-2 pb-6 fc-settings">
     <!-- Sticky tabs: operator-flagged 2026-05-25 — long Import / Maintenance
          panels pushed the tab strip out of the viewport, forcing a scroll-
          to-top just to change tab. AppShell's TopNav is 64px sticky, so the
@@ -125,3 +129,10 @@ watch(tab, (t) => {
   }
 })
 </script>
+
+<style scoped>
+/* Centered, capped measure for the whole Settings surface (tabs + panels). */
+.fc-settings {
+  max-width: 1140px;
+}
+</style>