Editor V2: Replace color-based collapse/expand indicators with arrow icons

[shopify-river]

Summary

Stop using background color as the indicator for a section being collapsed/expanded. Use standard arrow/chevron icons instead.

Rationale

From the Apr 21 CSOM sync: today collapsed windows get colored, which makes them look like section headers for the entire section, not like collapsed content. Standard disclosure arrows (▸ / ▾) communicate collapse state unambiguously without overloading color.

Acceptance

• Collapsed and expanded section headers use an arrow/chevron icon to indicate state.
• Background color is no longer used to signal collapse/expanded state.
• Applies across editor V2 collapsible sections (properties panel, pipeline details, etc.).

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Context: Captured from the Tangle / CSOM UX sync on Apr 21, 2026 (notes). Attendees: Matt Beaulne, Camiel van Schoonhoven, Maksym Yezhov.

Requested by Matt Beaulne matt.beaulne@shopify.com.