[Bug]: Plan composer footer pushes the Implement action out of bounds with long selections #1192

Before submitting

• I searched existing issues and did not find a duplicate.
• I included enough detail to reproduce or investigate the problem.

Area

apps/web

Steps to reproduce

1. Open the web app on a desktop-width viewport.
2. Go to a thread where the plan follow-up composer is shown ("Plan ready"), or tell the model to write a plan in Plan Mode in a new thread.
3. Select a long model name and enable a long reasoning label with Fast mode enabled.
4. Observe the footer layout near the Implement button.

Expected behavior

The Implement button should stay at the same place inside the composer, and the controls on the left should compress predictably without overlapping or getting pushed out of the container.

Actual behavior

Long selections push the Implement button out of the composer bounds.

(ignore the missing plan name, it was just a dummy plan with one word for demo)

Impact

Cosmetic issue

Version or commit

main @ 1310045

Environment

Zen 1.19.3b (Firefox 148.0.2)

Logs or stack traces

Screenshots, recordings, or supporting files

No response

Workaround

No response