Support for custom group colors

[matejur]

Motivation for features / changes

When comparing multiple runs across different models (e.g., training 5 models 10 times each), I often use the “Color runs by regex” feature.
This change adds the ability to manually assign colors to each regex group.

Technical description of changes

• Added a color picker for each regex group (similar to the existing picker next to individual runs).
• After defining the regex, users can optionally select a custom color for each group.
• Selected colors are stored in the main store and later applied when setting the colors of individual runs.

Screenshots of UI changes

Setting the regex

Selecting group color

After clicking Save

Detailed steps to verify changes work correctly (as executed by you)

1. Ensure there are at least two groups of runs (e.g., groupA and groupB as shown in the screenshots).
2. Enable Color runs by regex.
3. Use the regex (A|B) to create the groups.
4. Adjust the color for each group.
5. Click Save and confirm the runs update with the selected colors.

Alternate designs / implementations considered (or N/A)

My Angular experience is limited, so the implementation follows the existing approach used for onRunColorChange.
The colors are currently applied within the runGroupByChanged action.
If there is a more appropriate pattern for handling this, suggestions are welcome and I can update the implementation.
Tests can also be added if needed.