Summary

A layer's style custom attribute is stored as a raw CSS string, but it was being applied directly to React's style prop, which requires an object. On render paths that spread props without overriding style (e.g. images, inputs, iframes), the string leaked to the DOM and crashed server-side rendering with The style prop expects a mapping from style properties to values, not a string.

Changes

• Add parseStyleStringToObject() util that parses an inline CSS string into a React style object (splits on the first colon so values with colons like URLs survive; preserves --custom-properties verbatim).
• Add applyCustomAttributes() util that maps HTML attribute names to their JSX equivalents and, for a string style, parses and merges it into any existing computed style.
• Replace the four duplicated customAttributes loops (element + iframe) in LayerRenderer and LayerRendererPublic with the shared helper.

Test plan

• Add a custom attribute style = color: red; margin-top: 10px to a layer — element styles correctly, no console error on canvas
• Open the page in preview/published render — no server-side style prop error
• style value with a colon, e.g. background: url(https://x/y) — full URL preserved
• CSS variable, e.g. --accent: #f00 — preserved without camelCasing
• Custom attr style merges with the layer's existing computed styles