feat: add user_menu_links config for custom items in the user dropdown

[nzjrs]

Lets developers add links to the user menu, uses a similar paradigm to the way links can be added to the header.

The header has the symmetric header_links field; this adds the matching user_menu_links so the user menu is configurable too.

Each entry takes name, url, optional icon_url, optional display_name, and optional target (default _blank). icon_url is optional here (unlike HeaderLink, where it is required) because menu items are text-first; header items are icon-only buttons. Items render between the username label and the logout entry, with a separator inserted above logout when the list is non-empty. Each item uses Radix asChild over a real anchor, so middle-click and "open in new tab" behave correctly.

Backend: new UserMenuLink pydantic model, new optional UISettings field, and a commented example in the default config.toml template. Frontend: UserNav reads config.ui.user_menu_links via useConfig and resolves /public/... icon paths via ChainlitContext, matching the existing ButtonLink pattern.

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Summary by cubic

Adds configurable user_menu_links so teams can add custom items in the avatar dropdown. Mirrors header_links to let you link to account, billing, or docs without code changes.

• New Features
  • Backend: added UserMenuLink model and UISettings.user_menu_links, plus a commented example in the default config.toml.
  • Frontend: UserNav reads config.ui.user_menu_links via useConfig, renders items above Logout, resolves /public/... icons with ChainlitContext, and uses real anchors for proper new-tab behavior.
  • Types: extended @chainlit/react-client config types to include user_menu_links.

^{Written for commit 0b6cc76. Summary will update on new commits.}

[cubic-dev-ai]

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