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

[mengw15]

Task Summary

Part of #6870. The user-facing UI for managing and choosing warehouses, shown only when the
backend reports the feature is enabled (byoEnabled from GET /warehouse/status).

• Warehouse tab — a dashboard page to list, create (Local flavor), and delete your warehouses
  (user-warehouse component + a new warehouse.service.ts).
• On-canvas per-execution picker — a warehouse selector added to the computing-unit selection
  control (computing-unit-selection component), so you choose which warehouse a run writes to.
• Gating & routing — the Warehouse tab and the picker appear only when byoEnabled
  (dashboard component + routing); with the flag off the UI is unchanged.

Task Type

• Other (feature implementation)