settings_ui: Add fallback for `agent_ui_font_size` (#39782)

Closes https://github.com/zed-industries/zed/issues/39775

Release Notes:

- N/A
9ac010043cFinn Evers committed on 10/8/2025, 3:08:39 PM· committed by GitHubparentdd3b65f
1 file changedLine totals unavailable