Allow reordering gadget tabs within a workspace

[pol-cc]

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.