ui: fix removed reasoning menu in single model mode on desktop

[benbenbenbenbenben]

Overview

This PR restores the missing reasoning menu in the web UI on large/desktop layouts when running in single model mode. The regression appears to have been introduced in #27744 and fixes #27981

Additional information

N/A

Requirements

• I have read and agree with the contributing guidelines
• AI usage disclosure: Qwen 3.8 27B was used to find the offending commit, I accepted a single line change suggestion and ran all git and gh operations manually.

[allozaur]

@benbenbenbenbenben please run npm run format in tools/ui and update the failing Storybook test

[ServeurpersoCom]

Thanks for the fix. Could you also test it in router mode? The models selector already mounts the reasoning submenu there, so this needs to not end up showing it twice before it can be merged. Also you need to launch "npm run format" before commit.

[Remowylliams]

Very looking forward to this PR making it through. Reasoning menu was super useful.

[benbenbenbenbenben]

Thanks for the fix. Could you also test it in router mode? The models selector already mounts the reasoning submenu there, so this needs to not end up showing it twice before it can be merged. Also you need to launch "npm run format" before commit.

I've tested it. In router mode on large screens the reasoning menu is in the correct location and I confirmed it doesn't duplicated when the screen is narrow.

Model selector on large screens:

Model selector on small screens:

Reasoning in "+" menu on small screens: