Split installed MCP servers into user and workspace sections

The Chat Customizations editor currently shows all installed MCP servers in a single **Installed** section. This makes it difficult to understand whether a server is configured at the user or workspace level and to manage the correct configuration.

The MCP server list should match the other customization pages by splitting installed servers into **User** and **Workspace** sections.

This should use the MCP server source/scope information that is already available (and currently surfaced in the hover) so each server appears under the correct section.

## Expected behavior

- User-scoped MCP servers appear under **User**.
- Workspace-scoped MCP servers appear under **Workspace**.
- The generic **Installed** grouping is replaced by the scoped sections.