feat: default rounded brush lines and round cap toggle button

[imizgun]

Closes #590

This PR adds .with_line_cap(LineCap::Round) for brush tool as default drawing behavior, adds new button to the toolbar (Toggle Round Caps) next to the fill function. This button toggles round caps for arrow and line tools.

2026-07-21.23-57-22.mp4

[RobertMueller2]

Thanks for the PR. Now that we got this, I'm kinda wondering if we'll ever get a bug report that the setting has no effect on brush 😆

Could you please add the new option to the example config.toml and run make update-readme? And perhaps add a remark to the Brush tool section that it'll always use round caps?

I'm also wondering if there's better icons for this, re-using the circle isn't super ideal. But looking at the fluentui icons I don't have a good idea, either.

[imizgun]

If somebody will ask about brush not having this effect, I would personally deal with it :)
I've updated config.toml and README.md, in README.md noticed, that default-round-caps is true for brush always:

# Round caps for `arrow` and `line` tools (always true for `brush` tool)
default-round-caps = true

Maybe some other ideas on possible icons in toolbar?

[robertwidfen]

Tested and looks good to me.

Sorry for accidentally mentioning it in other ticket 🙃

[robertwidfen]

Regarding the icons - what do you (two) think about:

icons-rounded-caps.patch