Switch `.tsp._annotate.markup_gaps()` to use new
`.ui._style.get_fonts()` API for font size calc on client side and add
optional `show_txt: bool` flag to toggle gap duration labels (with
default `False`).
Also,
- replace `sgn` checks with named bools: `up_gap`, `down_gap`
- use `small_font.px_size - 1` for gap label font sizing
- wrap text creation in `if show_txt:` block
- update IPC handler to use `get_fonts()` vs direct `_font` import
(this commit msg was generated in some part by [`claude-code`][claude-code-gh])
[claude-code-gh]: https://github.com/anthropics/claude-code