[BYO-S3] Frontend: on-canvas per-execution warehouse picker

### Task Summary

Part of #6870, split out of #6933 so each frontend surface lands as one reviewable PR. The workspace-side selector for which warehouse a run writes to, mirroring the computing-unit picker's anatomy and behaviour.

- **Picker** — a dropdown next to the computing-unit selector (`computing-unit-selection` component): owner avatar per entry, a delete action per row, a create entry opening the shared create modal, and a list refresh on every open. Like the CU selector, it preselects the warehouse of the workflow's latest execution, falling back to the user's first warehouse; a manual pick survives refreshes.
- **Run gating** — with the feature enabled, every run needs a warehouse (#7751): with none selected the Run button becomes "Create Warehouse" and leads to the create modal, mirroring the Connect flow; the picker stays visible with zero warehouses so the create path is reachable.
- **Execution wiring** — the selected `warehouseId` rides the websocket execute request; `whId` on execution entries drives the preselect.

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