Add Register Update dialog to UpdatesDashboard

[bburda]

Summary

The Software Updates dashboard (PR #60) renders the gateway's /updates list and per-update actions (prepare, execute, automated toggle), but there is no UI entry point for the generic SOVD POST /updates call. Operators that want to drive an OTA campaign from the web UI currently have to curl the gateway directly.

Proposed solution

Add a Register Update button to the Updates dashboard toolbar that opens a dialog with:

• id (required, text)
• update_name (optional, text - defaults to id when blank so the gateway requirement is satisfied)
• automated (optional, checkbox)
• additional metadata (optional, JSON text area merged into the request body)

Client-side validation: required id, parse additional metadata as JSON before submit. Dialog calls a new registerUpdate Zustand store action that POSTs to /api/v1/updates.

Generic by design: no OTA-vendor-specific fields (everything custom goes in the JSON metadata field), so the same dialog works for Uptane, Mender, or any other UpdateProvider plugin behind the gateway.

Additional context

• Pairs with gateway-side /updates implementation that has been in place for several releases.
• Blocks an end-to-end demo: integration tests need a way to drive the register -> prepare -> execute flow through the UI.