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

backend/app/services/ml/heads.py

@@ -326,7 +326,10 @@ async def _current_heads(session: AsyncSession, embedding_version: str):
             {
                 "tag_id": r.tag_id,
                 "name": r.name,
-                "category": _CATEGORY.get(r.kind, "general"),
+                # System tags (wip/banner/editor) are kind=general but group under
+                # their OWN "system" suggestion category so the operator reviews
+                # them apart from content tags (they still train as general heads).
+                "category": "system" if r.is_system else _CATEGORY.get(r.kind, "general"),
                 "auto_apply_threshold": r.auto_apply_threshold,
                 "is_system": bool(r.is_system),
             }