feat(suggestions): group wip/banner/editor under a separate 'system' category

System tags are kind=general, so their suggestions previously landed in the
General group. Give them their own 'system' suggestion category so the operator
reviews them apart from content tags: _current_heads maps is_system heads to
category 'system' (still trained as general heads, still gated by the 0.65
floor). Frontend: CATEGORY_ORDER/LABELS gain 'system'; SuggestionsPanel renders
a 'System' group first (small, collapsible, open — false positives easy to spot
and reject); the typed-dropdown shows the shield icon for system entries. Safe:
system-tag suggestions always carry a canonical_tag_id, so the create-by-kind
path (which would send 'system' as a TagKind) is never hit.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CDgx8bQS5YrGRK76v8HUnM

tests/test_ml_suggestions.py

@@ -124,9 +124,11 @@ async def test_system_tag_surfaces_at_flat_floor_despite_high_threshold(db):
     await _head(db, tag.id, slot=0, suggest_threshold=0.9)
     await db.commit()
     sl = await SuggestionService(db).for_image(img.id)
+    # System tags surface under their OWN "system" category, not "general".
     assert any(
-        s.canonical_tag_id == tag.id for s in sl.by_category.get("general", [])
+        s.canonical_tag_id == tag.id for s in sl.by_category.get("system", [])
     )
+    assert not sl.by_category.get("general")
 
 
 @pytest.mark.asyncio
@@ -139,7 +141,7 @@ async def test_system_tag_below_floor_stays_hidden(db):
     await _head(db, tag.id, slot=0, suggest_threshold=0.9)
     await db.commit()
     sl = await SuggestionService(db).for_image(img.id)
-    assert not sl.by_category.get("general")
+    assert not sl.by_category.get("system")
 
 
 @pytest.mark.asyncio