[FEATURE] Implement collapsible() Component for UI Layout

[sandeepsalwan1]

name: "Collapsible UI Component"
about: Add the new collapsible() component to Preswald’s UI layout
title: "Implement collapsible() Component"
labels: enhancement
assignees: sandeepsalwan1

---

Related Issue

Closes #519

Description of Changes

I’ve added a brand‑new collapsible() component to Preswald so you can group UI elements into sections that users can expand or collapse. This helps keep things tidy and makes your interface feel more organized.

Type of Change

• Bug fix (non‑breaking change)
• New feature (non‑breaking change)
• Documentation update
• Breaking change
• New example
• Test improvement

Testing

Verified in the iris example app:

• Sections expand/collapse smoothly
• Child components render correctly
• Both open=true and open=false work as expected
• Styles match our design standards

Checklist

• Code follows project style guidelines
• Self‑review completed
• Comments added in tricky areas
• Documentation updated
• No new warnings introduced
• Examples run without errors
• Downstream modules updated as needed