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>

frontend/src/types/index.ts

@@ -246,8 +246,6 @@ export interface CreateApplicationDto {
   memoryLimit?: string;
   replicas?: number;
   port?: number;
-  clusterId?: string;
-  poolId?: string;
   customDomain?: string;
   // Optional services
   enableRedis?: boolean;