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/components/ui/ConfirmDialog.tsx

@@ -0,0 +1,46 @@
+import type { ReactNode } from "react";
+import { Button } from "@/components/ui";
+
+export function ConfirmDialog({
+  title,
+  message,
+  confirmLabel = "Confirm",
+  danger = false,
+  busy = false,
+  onConfirm,
+  onCancel,
+  children,
+}: {
+  title: string;
+  message?: string;
+  confirmLabel?: string;
+  danger?: boolean;
+  busy?: boolean;
+  onConfirm: () => void;
+  onCancel: () => void;
+  children?: ReactNode;
+}) {
+  return (
+    <div
+      className="fixed inset-0 z-50 flex items-center justify-center bg-black/50 p-4"
+      onClick={() => !busy && onCancel()}
+    >
+      <div
+        className="w-full max-w-md rounded-xl border border-slate-200 bg-card p-5 shadow-xl dark:border-slate-700 dark:bg-card-dark"
+        onClick={(e) => e.stopPropagation()}
+      >
+        <h2 className="text-lg font-semibold">{title}</h2>
+        {message && <p className="mt-2 text-sm text-slate-500">{message}</p>}
+        {children && <div className="mt-3">{children}</div>}
+        <div className="mt-4 flex justify-end gap-2">
+          <Button variant="outline" onClick={onCancel} disabled={busy}>
+            Cancel
+          </Button>
+          <Button variant={danger ? "danger" : "primary"} onClick={onConfirm} loading={busy}>
+            {confirmLabel}
+          </Button>
+        </div>
+      </div>
+    </div>
+  );
+}