Expose font sizing control for `pg.TextItem` annotations thru the
annot-ctl API. Default to `_font.font.pixelSize() - 3` when no
size provided.
Also,
- thread `font_size` param thru IPC handler in `serve_rc_annots()`
- apply font via `QFont.setPixelSize()` on text item creation
- add `?TODO` note in `markup_gaps()` re using `conf.toml` value
- update `add_text()` docstring with font_size param desc
(this patch was generated in some part by [`claude-code`][claude-code-gh])
[claude-code-gh]: https://github.com/anthropics/claude-code