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/package.json

@@ -1,7 +1,7 @@
 {
   "name": "stackpilot-frontend",
   "private": true,
-  "version": "0.8.0",
+  "version": "0.9.0",
   "type": "module",
   "scripts": {
     "dev": "vite",