[Bug]: Composer controls overlap when plan sidebar is open on narrow windows

[UtkarshUsername]

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 T3 Code and go to/start a thread.
2. Tell the AI to write a plan in plan mode.
3. Open the plan sidebar.
4. Resize and make the window smaller.

Expected behavior

Proposed Solution:

Make the plan sidebar use the same responsive behavior as the diff panel.

• Render inline on wider screens.
• Switch to a right-side sheet at the same breakpoint the diff panel uses.
• Keep the existing open/close and auto-open behavior for active plans.
• Reuse the same sheet sizing so both right-side panels feel consistent.

Actual behavior

It isn't responsive and make the chat windows very small. The composer controls overlap.

(Video shared by Julius)
https://github.com/user-attachments/assets/6d83f027-ec3e-4026-8c8a-6130cc70a932

Impact

Minor bug or occasional failure

Version or commit

main @ b500a36

Environment

Zen 1.19.3b (Firefox 148.0.2)

Logs or stack traces

Screenshots, recordings, or supporting files

No response

Workaround

Close the plan sidebar on small windows.