Desktop: "Duplicate agent" dialog has no "Run on" selector and always creates a local agent

**Describe the bug**

Duplicating an agent opens a create dialog without the "Run on" selector. Creating an agent from scratch has it, so you can pick where the agent runs (this machine or a provider). The duplicate dialog does not, and it still says "Create agent" and starts the new agent right away — always locally, even when the agent being duplicated runs on a provider.

So duplicating a provider-hosted agent silently gives you a local one, with no way to choose during creation.

**Steps to reproduce**
1. Create an agent and set "Run on" to a remote provider.
2. Agents screen, open the agent's ⋯ menu, click "Duplicate".
3. The dialog has name, prompt, model and the rest, but no "Run on" section.
4. Click "Create agent" — the copy is created and started on this machine.

**Expected behavior**

The duplicate dialog offers the same "Run on" choice as the create dialog, pre-filled with the source agent's location.

**Version and platform**
- Buzz version: 0.5.23
- OS: macOS

**Logs / additional context**

Searched for an existing report on the duplicate flow and the missing "Run on" selector; none found.

Start at the Agents screen's ⋯ menu and follow the Duplicate flow, then compare its create dialog with the dialog used when creating an agent from scratch. Done means the duplicate dialog offers the same "Run on" choice, pre-filled with the source agent's location, and no longer always starts a local copy.

Written by the indexing model from the issue text.