Replace full-widget background coloring with color indicator button

[Copilot]

Stratigraphic column text was unreadable when unit colors (e.g., dark blue, black) colored the entire widget background.

Changes

• UI: Added 40px buttonColour widget as column 0, shifts existing fields right
• Styling: Color now applied only to button via _update_colour_button() instead of widget-level setStyleSheet()
• Interaction: Button click opens color picker; removed mouseDoubleClickEvent() handler

# Before: entire widget colored
self.setStyleSheet(f"background-color: {self.colour};")

# After: only button colored  
def _update_colour_button(self):
    self.buttonColour.setStyleSheet(
        f"background-color: {self.colour}; border: 1px solid #999;"
    )

No API changes. Existing data and signals unchanged.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Change stratigraphic column colouring</issue_title>
<issue_description>### Feature description

Sometimes hard to read the stratigraphic column names with contrast

Additional context

No response</issue_description>

<agent_instructions>Change from colouring the whole list item to just having a colour button on the stratigraphic unit. </agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes Change stratigraphic column colouring #78

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