feat(FN-2681): add Vitest kill controls in system stats

- Add dashboard API endpoint and server route to terminate Vitest worker processes safely
- Extend legacy API client with killVitestProcess support for frontend invocation
- Add SystemStatsModal UI controls and styling for triggering Vitest kill actions, including mobile width stability fixes
- Expand route and modal tests to cover new controls and behavior
- Add changeset and architecture docs updates for the new Vitest kill capability

packages/dashboard/app/api/legacy.ts

@@ -4896,12 +4896,25 @@ export interface TaskStatsSnapshot {
 export interface SystemStatsResponse {
   systemStats: SystemStatsSnapshot;
   taskStats: TaskStatsSnapshot;
+  vitestProcessCount?: number;
+  vitestLastAutoKillAt?: string | null;
+}
+
+export interface KillVitestResponse {
+  killed: number;
+  pids: number[];
 }
 
 export function fetchSystemStats(projectId?: string): Promise<SystemStatsResponse> {
   return api<SystemStatsResponse>(withProjectId("/system-stats", projectId));
 }
 
+export function killVitestProcesses(projectId?: string): Promise<KillVitestResponse> {
+  return api<KillVitestResponse>(withProjectId("/kill-vitest", projectId), {
+    method: "POST",
+  });
+}
+
 /** Fetch unified activity feed */
 export function fetchActivityFeed(options?: FeedOptions): Promise<ActivityFeedEntry[]> {
   const params = new URLSearchParams();