fix(ui): restore sticky positioning on diffs header

[nexxeln]

summary

• Restores position: sticky on the "Modified files" header in the session turn diffs section
• Regression from feat(ui): redesign modified files section in session turn #20348 which replaced the Collapsible wrapper with a plain header div but dropped the sticky CSS rules

testing

• Open a session with enough file edits to scroll, verify the "N Changed files" header sticks to the top while scrolling through diffs