Phase 9: network management + stack delete in UI (0.9.0)

- Networks: network_service (list w/ subnet/containers/in-use/owning-stack,
  create bridge/macvlan/ipvlan/overlay + optional subnet/gateway/internal,
  delete with default-network guard, prune) + routers/networks.py; real
  Networks page replaces the placeholder.
- Fix: local stacks can now be deleted from the UI — Delete button on stack
  detail (with optional keep-files-on-disk) and a trash action on stack cards,
  via a shared ConfirmDialog. (Backend DELETE existed; no UI surfaced it.)

Verified: py_compile, frontend tsc build, live network list smoke test
(defaults flagged, compose nets + in-use detected); main 104 routes.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/App.tsx

@@ -11,7 +11,7 @@ import { Images } from "@/pages/Images";
 import { Templates } from "@/pages/Templates";
 import { Settings } from "@/pages/Settings";
 import { Audit } from "@/pages/Audit";
-import { Networks } from "@/pages/Placeholder";
+import { Networks } from "@/pages/Networks";
 import { useAuthStore } from "@/store/auth";
 import { useThemeStore } from "@/store/theme";