tab: Add optional overflow edge fades

[ihavecoke]

Summary

Overflowing tab rows can now fade their clipped ends into the surrounding surface. Fades appear only where tabs are hidden and leave fixed controls and pointer events intact.

 TabBar::new("document-tabs")
     .outline()
+    .with_edge_fade(cx.theme().background)

Changes

• Read scroll offset and actual first/last tab bounds during paint, including the first layout and explicit reveals.
• Exclude the trailing spacer from overflow hints and keep prefixes, suffixes and menus clear.
• Document the opt-in API in English and Chinese.

Evidence

• Before: tabs and outlines ended abruptly at the scroll viewport.
• After: all 13 TabBar tests pass, including edge visibility, resizing, explicit reveals and clicks beneath fades.
• cargo clippy -p gpui-component --all-targets --no-default-features -- -D warnings passes.
• cargo +nightly fmt --all -- --check passes.
• Allsum integration: UI consistency checks, compilation and all 620 Allsum tests pass.

Public API

gpui-component

TabBar::with_edge_fade(self, color: impl Into<Hsla>) -> Self: optionally fade overflowing tab edges into the supplied surface color. Existing callers retain their current appearance.

Context

The preview toolbar needs a visible hint that more document tabs can be reached by scrolling. Implementation, tests and documentation were AI-assisted.

Merge Danger

Door: two-way

Blast Radius: tabs

The effect is opt-in; no data or migration changes.