feat: add Environment admin UI page

SplitPane with create/edit/delete, production flag toggle,
enabled/disabled toggle. Follows existing admin page patterns.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ui/src/api/queries/admin/environments.ts

@@ -0,0 +1,63 @@
+import { useQuery, useMutation, useQueryClient } from '@tanstack/react-query';
+import { adminFetch } from './admin-api';
+
+export interface Environment {
+  id: string;
+  slug: string;
+  displayName: string;
+  production: boolean;
+  enabled: boolean;
+  createdAt: string;
+}
+
+export interface CreateEnvironmentRequest {
+  slug: string;
+  displayName: string;
+  production: boolean;
+}
+
+export interface UpdateEnvironmentRequest {
+  displayName: string;
+  production: boolean;
+  enabled: boolean;
+}
+
+export function useEnvironments() {
+  return useQuery({
+    queryKey: ['admin', 'environments'],
+    queryFn: () => adminFetch<Environment[]>('/environments'),
+  });
+}
+
+export function useCreateEnvironment() {
+  const qc = useQueryClient();
+  return useMutation({
+    mutationFn: (req: CreateEnvironmentRequest) =>
+      adminFetch<Environment>('/environments', {
+        method: 'POST',
+        body: JSON.stringify(req),
+      }),
+    onSuccess: () => qc.invalidateQueries({ queryKey: ['admin', 'environments'] }),
+  });
+}
+
+export function useUpdateEnvironment() {
+  const qc = useQueryClient();
+  return useMutation({
+    mutationFn: ({ id, ...req }: UpdateEnvironmentRequest & { id: string }) =>
+      adminFetch<Environment>(`/environments/${id}`, {
+        method: 'PUT',
+        body: JSON.stringify(req),
+      }),
+    onSuccess: () => qc.invalidateQueries({ queryKey: ['admin', 'environments'] }),
+  });
+}
+
+export function useDeleteEnvironment() {
+  const qc = useQueryClient();
+  return useMutation({
+    mutationFn: (id: string) =>
+      adminFetch<void>(`/environments/${id}`, { method: 'DELETE' }),
+    onSuccess: () => qc.invalidateQueries({ queryKey: ['admin', 'environments'] }),
+  });
+}