Split the shared `AgentFontSize` in-memory global into separate
`AgentBufferFontSize` and `AgentUiFontSize` globals. Previously, both
`adjust_agent_ui_font_size` and `adjust_agent_buffer_font_size` read
from and wrote to the same global, so calling them back-to-back in
`handle_font_size_action` applied the delta twice.

Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [ ] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [ ] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Closes #53368.

Release Notes:

- Fixed Agent Panel font size incrementing by 2px instead of 1px