setting: Add optional footers outside group surfaces

[AprilNEA]

Description

Settings sometimes need supporting text below a group, outside its background or border. A custom SettingItem still renders inside that surface.

This adds an optional footer to GroupBox and exposes it through SettingGroup. The footer uses the variant's default horizontal content inset; callers control its typography and color. In Settings, it scrolls and filters with its group without adding a sidebar entry or independently searchable item.

Existing groups without footers are unchanged. Includes gallery examples, English/Chinese documentation, and a search-visibility regression test. No gpui-base changes.

Public API

gpui-component

GroupBox::footer(self, footer: impl IntoElement) -> Self

Adds supporting content below and outside the group's surface.

SettingGroup::footer<F, E>(self, footer: F) -> Self
where
    E: IntoElement,
    F: Fn(&mut Window, &mut App) -> E + 'static

Renders a group footer using the current window and application context.

Both additions are optional and backward compatible.

Screenshot

How to Test

Passed on Linux with Rust 1.98.1:

cargo fmt --all -- --check
cargo test -p gpui-component --locked
cargo clippy -p gpui-component -p gpui-component-story \
  --all-targets --no-default-features --locked -- -D warnings
cargo build -p gpui-component-story --no-default-features --locked
python3 script/check-ai docs
git diff --check

• 580 tests passed, including footer visibility when a group is filtered out and restored.
• Ran the GroupBox and Settings stories under X11/OpenGL. Inspected Normal, Outline, and Fill variants, Font navigation, and a narrower window.
• To reproduce, open the Settings story, change Group Variant, and select Font.
• Not run locally: macOS/Windows rendering or the full workspace CI matrix.

[Copilot]

Copilot review overview

🔵 Needs a closer look

Address the Outline inset issue and add GroupBox variant coverage.

Review effort: Lite
Findings: None

What changed in this PR

Adds optional footers to GroupBox and SettingGroup, keeping footer content outside group surfaces while preserving settings filtering and scrolling.

Changes:

• Added GroupBox::footer and SettingGroup::footer APIs.
• Added search-visibility regression coverage.
• Updated stories and English/Chinese documentation.

File	Reviewed changes
website/​zh-CN/​component/​settings.md	Documents SettingGroup footers in Chinese.
website/​zh-CN/​component/​group-box.md	Documents GroupBox footers in Chinese.
website/​component/​settings.md	Documents the SettingGroup footer API.
website/​component/​group-box.md	Documents the GroupBox footer API.
crates/​story/​src/​stories/​settings_story.rs	Demonstrates settings footers.
crates/​story/​src/​stories/​group_box_story.rs	Demonstrates GroupBox footers.
crates/​component/​src/​setting/​tests.rs	Tests footer visibility during filtering.
crates/​component/​src/​setting/​group.rs	Exposes callback-based setting footers.
crates/​component/​src/​group_box.rs	Adds variant-aware footer rendering; the Outline inset should account for the surface border, with automated coverage for all variants.

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