Allow reordering gadget tabs within a workspace

## Problem

In a workspace with several gadgets, the gadget tabs in the top bar always appear in **creation order**, and there is no way to rearrange them. For workspaces used as day-to-day dashboards by "use"-role collaborators, the tab order matters (e.g. putting the most-used gadget first, grouping related ones), and today the only way to change it is to delete and recreate gadgets in the desired order — which destroys their Durable Object state.

## Where the order comes from today

- The "use" view's picker (`packages/workshop-frontend/src/GadgetUseView.tsx`) renders `gadgets` in the order received; the editor view behaves the same.
- That list is fed by `subscribeToWorkpieces()` in `packages/workshop-backend/src/overseer.ts`, which iterates `this.storage.gadgets.list()`.
- `typed-storage`'s `list()` iterates keys in ascending order, so the tab order is effectively gadget-id (creation) order.
- `GadgetRecord` / `WorkpieceSummary` carry no ordering field, and there is no API to move a workpiece.

## Proposal

Let workspace owners/editors reorder gadgets, and persist that order:

1. Add an optional `order` (sort key) field to `GadgetRecord`, defaulting to creation order for existing records.
2. Expose a small overseer API to move a gadget (e.g. `setWorkpieceOrder(id, beforeId | afterId)` or a fractional-index sort key), owner/edit roles only.
3. Sort by `(order, id)` in `subscribeToWorkpieces()` so all subscribers (editor and "use" views) see the same order.
4. Frontend: drag-and-drop on the tab strip, or simple move-left/move-right controls in the tab context menu as a first step.

A fractional/lexicographic sort key would avoid rewriting sibling records on every move; happy to discuss alternatives.

## Notes

- No migration should be needed: records without `order` can sort by id, preserving today's behavior.
- We operate deployed instances of Cloudflare OS for clients and this comes directly from end-user feedback on multi-gadget dashboard workspaces.

Start with packages/workshop-frontend/src/GadgetUseView.tsx and packages/workshop-backend/src/overseer.ts, then trace GadgetRecord and WorkpieceSummary. Decide how persisted ordering and the owner/editor move API should work, including the existing-record fallback. Done means both editor and use views receive the same reordered gadget tabs without destroying Durable Object state.

Written by the indexing model from the issue text.