[BYO-S3] Frontend: warehouse dashboard tab

### Task Summary

Part of #6870. The dashboard page for managing warehouses, mirroring the computing-unit tab's structure and look, shown only when the backend reports the feature enabled (`warehouseEnabled` from `GET /warehouse/status`; with the flag off the UI is unchanged).

- **Warehouse tab** — list, create (Local flavor), and delete your warehouses: `user-warehouse` container + `user-warehouse-list-item` row cards (same virtual-scroll list, hover actions, and row anatomy as the computing-unit tab), a shared create modal, and a read-only details dialog (name, owner, flavor, created).
- **Shared plumbing** — `warehouse.service.ts` (status/create/delete + the execution-time pick that the picker (#7817) consumes) and `WarehouseActionsService` owning the delete confirmation, mirroring `ComputingUnitActionsService`.
- **Gating & routing** — the sidebar entry and route appear only when `warehouseEnabled` (`dashboard` component + routing).

The on-canvas per-execution picker is split out to #7817 so each lands as one reviewable PR.

### Task Type
- [x] Other (feature implementation)