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/dashboard.service.ts

@@ -142,7 +142,7 @@ export class DashboardService {
       action: row.action,
       entityType: row.entityType,
       entityId: row.entityId,
-      details: row.details,
+      details: row.details as Record<string, unknown> | null,
       userId: row.userId,
       createdAt: row.createdAt.toISOString(),
     }));