Fix create session page scrolling

[lidaxian121]

Summary

• keep the create session header outside the scrolling region
• move the form content into a height-constrained inner scroll container
• ensure the content pane can scroll when agent/model options make the page taller

Problem

On the create session page, choosing agents like Codex adds enough options that the lower action buttons can become unreachable because the page cannot scroll vertically.

Testing

• local manual verification of the create session page layout
• confirmed the scroll region starts below the header and can scroll through the full form

[github-actions]

Review mode: initial. No issues found in modified lines. Residual risk: layout regressions on small screens/Telegram webview; tests not run.