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

tests/test_importer.py

@@ -116,6 +116,30 @@ def test_transparent_filter(importer, import_layout):
     assert result.skip_reason == SkipReason.too_transparent
 
 
+def test_single_color_filter(importer, import_layout):
+    """The skip_single_color setting existed since FC-2 but was never wired
+    (the audit module's docstring said so); wired 2026-07-02 using the same
+    canonical predicate as the Cleanup audit. Solid fill skips when enabled,
+    imports when disabled (the default)."""
+    from PIL import Image as PILImage
+
+    import_root, _ = import_layout
+    solid = import_root / "solid.png"
+    solid.parent.mkdir(parents=True, exist_ok=True)
+    PILImage.new("RGB", (100, 100), (12, 34, 56)).save(solid)
+
+    importer.settings.skip_single_color = True
+    importer.settings.single_color_threshold = 0.95
+    result = importer.import_one(solid)
+    assert result.status == "skipped"
+    assert result.skip_reason == SkipReason.single_color
+
+    importer.settings.skip_single_color = False
+    solid2 = import_root / "solid2.png"
+    PILImage.new("RGB", (100, 100), (200, 10, 10)).save(solid2)
+    assert importer.import_one(solid2).status == "imported"
+
+
 def test_unsupported_extension(importer, import_layout):
     # FC-2d-iii: non-media is no longer skipped — it's captured as a
     # PostAttachment so nothing a post contained is lost.