refactor(admin): remove Pillow as selectable renderer, restructure admin page

- Backend: VALID_RENDERERS = {"blender"} only; remove pillow from renderer-status response
- Frontend: remove renderer picker (pillow/blender buttons) — Blender is the only renderer
- Blender options always visible, grouped into Render Quality / Performance / Output sections
- Maintenance buttons in 2-column grid with descriptions
- Page reorder: Pricing Summary → Users → Blender Settings → Render Templates →
  Asset Libraries → Output Types → Pricing Tiers → SMTP → Templates
- Pillow code kept internally as fallback (not exposed in UI)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/api/routers/admin.py

@@ -16,7 +16,7 @@ from app.utils.auth import require_admin, hash_password
 
 router = APIRouter(prefix="/admin", tags=["admin"])
 
-VALID_RENDERERS      = {"pillow", "blender"}
+VALID_RENDERERS      = {"blender"}
 VALID_ENGINES        = {"cycles", "eevee"}
 VALID_FORMATS        = {"jpg", "png"}
 VALID_STL_QUALITIES  = {"low", "high"}
@@ -459,7 +459,6 @@ async def renderer_status(
     blender_available = is_blender_available()
     blender_bin = find_blender()
     return {
-        "pillow": {"available": True, "note": "Built-in (always available)"},
         "blender": {
             "available": blender_available,
             "note": (