Feature: configurable default pen color

Summary

Two related requests for annotation colors:

1. A setting for which color drawing starts in.
2. A way to change the actual RGB values behind the color keys.

Current behavior

Drawing always starts in red. Pressing R/G/B/O/Y/P switches color, but the
choice is not persisted — every new drawing session starts red again.

The eight color keys are also fixed to the system palette. There is no way
to change what shade R or B produces.

Why this matters

Pen width is already configurable through preferences, so a user can make
the pen thinner and it stays thinner. Color has no equivalent, which is a
surprising asymmetry — the two settings sit side by side conceptually.

Red is a poor default in some contexts. It is the conventional color for
errors and diffs, so annotating a terminal or an IDE in red can be
ambiguous. It also has low contrast against several common IDE themes.

For the second request: presenters working under brand guidelines need
specific shades, not approximations. Being able to map the existing keys to
custom hex values would cover that without adding new UI complexity or new
shortcuts.

Suggested shape

Both could follow the pattern the break timer already uses for its text and
background colors — an RGB value stored in preferences with a color picker
in settings.

For the default: one preference naming the starting color.

For the palette: six or eight RGB preferences, one per color key, defaulting
to the current system colors so nothing changes for existing users.

Whether the selected color should also persist across sessions rather than
resetting is a separate question, but worth considering alongside this.

Happy to open a PR if this direction sounds useful.

Start by locating the existing pen-width preferences and the break timer's text and background color settings to understand how preferences and color pickers are implemented. Clarify whether this issue covers the default color, customizable key mappings, selected-color persistence, or all three, then define acceptance criteria for the preference defaults and existing-user behavior.