Fix WASM highlighter stub mismatch in release docs build

[Copilot]

Description

The Release Docs workflow was failing in the Build Story Web (WASM) step because the WASM-only HighlightThemeStyle stub had drifted from the native struct shape. This patch restores parity by adding the missing gutter background field used by the input renderer.

• Root cause

  • crates/ui/src/input/element.rs now reads highlight_theme.style.editor_gutter_background
  • crates/ui/src/highlighter/wasm_stub.rs did not define that field, so wasm32-unknown-unknown builds failed

• Change

  • Add editor_gutter_background: Option<gpui::Hsla> to the WASM stub
  • Preserve the serialized theme key with #[serde(rename = "editor.gutter.background")]

#[serde(rename = "editor.gutter.background")]
pub editor_gutter_background: Option<gpui::Hsla>,

Screenshot

Before	After
WASM build fails with no field editor_gutter_background on type wasm_stub::HighlightThemeStyle	WASM highlighter stub matches native theme style and the build proceeds

Break Changes

None.

How to Test

1. Run cargo build --target wasm32-unknown-unknown -p gpui-component
2. Optionally run the release-docs build path from crates/story-web:
   • make build-prod

Checklist

• I have read the CONTRIBUTING document and followed the guidelines.
• Reviewed the changes in this PR and confirmed AI generated code (If any) is accurate.
• Passed cargo run for story tests related to the changes.
• Tested macOS, Windows and Linux platforms performance (if the change is platform-specific)