fix: remediate dashboard API integration review blockers (#459)

- Fix race condition: guard useEffect when workspaceId is null, prevent
  infinite loading state by setting isLoading=false on null workspace
- Fix TypeScript strict typing: @Workspace() returns string|undefined,
  controller now matches with BadRequestException guard
- Narrow details DTO type from unknown to Record<string, unknown>|null
- Add error state UI for API fetch failures
- Add error-path test with static mock import pattern

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/api/src/dashboard/dto/dashboard-summary.dto.ts

@@ -17,7 +17,7 @@ export class RecentActivityDto {
   action!: string;
   entityType!: string;
   entityId!: string;
-  details!: unknown;
+  details!: Record<string, unknown> | null;
   userId!: string;
   createdAt!: string;
 }