feat: plot data average as dots with across-replica error bars

[ahmadomira]

Summary

Add an option to display the averaged data as dots instead of the current average line, via an exclusive Line/Dots toggle in Plot Style, with a color picker for the averaged series. When dots are selected with replicas hidden and error bars enabled, show the average dots with ±1 SD across-replica error bars, reusing the existing error-bar width/cap style.

Source

Reported via the fitting-app feedback channel.

Acceptance criteria

• Plot Style exposes an exclusive Line/Dots toggle for the averaged series plus a color picker for it.
• Dots mode + replicas hidden + error bars on → average dots with ±1 SD across-replica bars (existing bar style reused).
• Test for the across-replica SD computation if added; uv run ruff check / format clean.

Scope & file boundaries

Package E. Touch:

• gui/plotting/plot_style.py
• gui/plotting/plot_widget.py
• gui/plotting/colors.py
• core/data_processing/plotting.py

Parallelization

Package E — start only after Package D (the loading-fit bug fix) PR has merged; branch from the updated main so the shared plotting files already include that fix.