feat(deploy): remove cluster allocation section from app creation

Drop the admin-only "cluster assignment" UI from the resources &
config step of the deploy wizard and the related backend override.
App placement is now always decided automatically by the allocator.

- frontend: remove cluster/pool selection block, review-step cluster
  row, clusterMode state, public cluster/pool queries, and clusterId/
  poolId from CreateApplicationDto
- backend: drop clusterId/poolId override from the create DTO and
  simplify ApplicationsService.create to always auto-allocate; widen
  selectClusterForApplication param to keep the fallback path working

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/src/clusters/clusters.service.ts

@@ -266,7 +266,7 @@ export class ClustersService implements OnModuleInit, OnModuleDestroy {
   }
 
   async selectClusterForApplication(
-    dto: CreateApplicationDto,
+    dto: CreateApplicationDto & { poolId?: string },
     userId: string,
     options: {
       excludeClusterIds?: string[];