feat(scenarios): link scenarios to a default environment

- add optional ManyToOne relation from scenario to environment entity
- expose environmentId in create/update DTOs, service, and MCP tools
- pre-select linked environment in run modals on detail and list pages
- add environment selector to create/edit scenario forms
- show linked environment as a navigable link on scenario detail page

client/src/api/client.ts

@@ -182,13 +182,13 @@ export const scenarios = {
   get(id: string): Promise<Scenario & { steps: ScenarioStep[] }> {
     return request(`/scenarios/${id}`);
   },
-  create(name: string, description?: string): Promise<Scenario> {
+  create(name: string, description?: string, environmentId?: string): Promise<Scenario> {
     return request('/scenarios', {
       method: 'POST',
-      body: JSON.stringify({ name, description }),
+      body: JSON.stringify({ name, description, environmentId }),
     });
   },
-  update(id: string, patch: Partial<Pick<Scenario, 'name' | 'description'>>): Promise<Scenario> {
+  update(id: string, patch: Partial<Pick<Scenario, 'name' | 'description' | 'environmentId'>>): Promise<Scenario> {
     return request(`/scenarios/${id}`, {
       method: 'PATCH',
       body: JSON.stringify(patch),