feat(ia): wave 1 — Import tab dissolves, Maintenance regroups by system, one extension home

Settings IA per the approved A3 design (the old layout was the two-app merge
fossilized):
- Import tab retired: ImportTriggerPanel + ImportTaskList deleted (manual
  /import scans stay API-level; imports arrive via downloads/extension, heal
  via the Layer-2 auto-refetch sweep, and show in Activity). ImportFiltersForm
  moves to Maintenance → 'Ingestion & filters' and loads its own settings; the
  import store shrinks to settings-only (no remaining consumers of the
  scan/task-list machinery). Overview's pending banner now points at Activity.
- Maintenance regrouped: Ingestion & filters / GPU agent & embeddings
  (GpuAgent, Failed processing, CPU embedding backfill) / Tagging (sliders,
  Heads, Aliases) / Library health (MissingFiles, Thumbnails, DB, Archive
  re-extract demoted last) / Storage.
- One extension home: BrowserExtensionCard moves from Settings → Overview to
  Subscriptions → Settings, above the API key bar it authenticates.
- Single-color import filter WIRED: skip_single_color/threshold existed since
  FC-2 but nothing read them (the audit module's docstring said as much) —
  now enforced on both import paths via the audit's canonical predicate
  (tolerance 30, matching the Cleanup card default; animated images exempt
  like the transparency check). Default stays off; test added.
- Dead weight: PlaceholderView (zero refs) and the permanently-disabled
  'Export failed logs (CSV — v2)' menu stub deleted; stale docs fixed
  (celery queue docstring, threshold comment citing retired tasks, ml
  package docstring, HeadsCard 'replaces Camie' blurb).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CDgx8bQS5YrGRK76v8HUnM

frontend/src/components/subscriptions/SettingsTab.vue

@@ -1,12 +1,18 @@
 <template>
   <div>
+    <!-- One extension home (2026-07-02): install/manifest card (moved here
+         from Settings → Overview) + the API key bar it authenticates with.
+         The extension feeds THIS view — cookies + one-click sources — so its
+         setup lives with the rest of the ingestion config. -->
+    <h3 class="text-h6 mb-3">Browser extension</h3>
+    <BrowserExtensionCard class="mb-3" />
     <ExtensionKeyBar class="mb-4" />
 
     <v-alert v-if="credentialsStore.error" type="error" variant="tonal" closable class="mb-4">
       {{ String(credentialsStore.error) }}
     </v-alert>
 
-    <h3 class="text-h6 mb-3">Platform credentials</h3>
+    <h3 class="text-h6 mb-3 mt-6">Platform credentials</h3>
     <v-row>
       <v-col
         v-for="p in platformsStore.list"
@@ -179,6 +185,7 @@ import { onMounted, reactive, ref, watch } from 'vue'
 import { usePlatformsStore } from '../../stores/platforms.js'
 import { useCredentialsStore } from '../../stores/credentials.js'
 import { useImportStore } from '../../stores/import.js'
+import BrowserExtensionCard from '../settings/BrowserExtensionCard.vue'
 import ExtensionKeyBar from '../credentials/ExtensionKeyBar.vue'
 import CredentialUploadDialog from '../credentials/CredentialUploadDialog.vue'
 import CredentialCard from './CredentialCard.vue'