fix(editor): darken explorer tabs and fade layers scroll

[DavidBabinec]

What changed

• Added a recessed active surface option to SegmentedControl for dark panel tab strips.
• Applied that darker active state to the Explorer panel tab switcher.
• Added a top fade under the Layers search row so scrolled layer rows blend under the fixed chrome.
• Added a focused regression test for the Explorer tab surface and Layers search fade hooks.

Why

The newly consolidated Explorer mode switcher rendered the active tab with a lighter gray fill, but the intended editor chrome treatment is the darker/black active surface. The Layers list also met the fixed search row with a hard edge while scrolling.

Impact

The Explorer panel active mode now reads as the darker selected state, while other segmented controls keep their existing default active surface unless they opt into the recessed variant.

Verification

• bun test ./src/__tests__/ui/segmentedControlChrome.test.tsx
• bun run lint
• bun run build
• bun test
• bunx react-doctor@latest --verbose --scope changed --base origin/main
• Browser smoke with disposable SQLite data: active Layers background computed as rgb(27, 27, 27) and Layers search fade rendered as black-to-transparent.