Add dedicated toggle button with blue background for control panel visibility

[Copilot]

The control panel's expand/collapse chevron was not discoverable—users couldn't tell the panel was collapsible or locate the toggle when collapsed.

Changes

Toggle button styling:

• Dedicated button area (45px × 60px) with light blue background (rgba(59, 130, 246, 0.25))
• Positioned at top-left corner when collapsed, moves to right edge of panel (320px) when expanded
• Borders, rounded corner, and backdrop blur separate it visually from panel content
• White chevron text for contrast

Interaction:

• Hover state increases opacity to 0.4 and enhances shadow
• Smooth 0.3s transitions between states

Before/After

Before: Chevron floated mid-screen, transparent background, blue text

After: Dedicated button area, always visible in top-left

Original prompt

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This section details on the original issue you should resolve

<issue_title>Make the control panel's expand/collapse feature more obvious</issue_title>
<issue_description>I didn't know the control panel could be expanded/collapsed at first. It was only when the chevron aligned next to the "Item Intensity" heading that I realized what it was. The hovering chevron when it is collapsed could also be improved.

<agent_instructions>Perhaps the < and > should be in their own dedicated space on the left side of the bar, that is highlighted in a slightly different colour, perhaps a light blue, to make it clear that it isn't part of any of the content within the bar.
When collapsed, this other bar on the edge would still be partially visible to make it clear that it can be expanded</agent_instructions>

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

• Fixes Make the control panel's expand/collapse feature more obvious #33

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[addshore]

I tested this out and it looks good to me and works well