fix(computer-use): surround cursor with a soft contour shadow

[bobleer]

Summary

Replace the Computer Use cursor's directional drop shadow with a centered shadow around the entire outline. The shadow is darker beside the white border and fades into a wider, softer outer halo in the macOS/Windows native overlays and Web preview.

Type and Areas

Type: UI/UX, bug fix

Areas: desktop Computer Use (macOS/Windows), Web UI preview

Motivation / Impact

The directional shadow could appear concentrated above the pointer. Two centered shadow layers now provide contrast around every edge, with a stronger contact shadow and a broader falloff. The cursor size, white border, hotspot, input routing, and animation remain unchanged.

The Windows raster test now checks symmetry and decreasing black opacity outside all four sides to catch directional or missing-edge regressions.

Verification

• node scripts/test-macos-control-pointer.mjs — passed, including passive overlay lifetime, occlusion, movement, and focus preservation.
• Inspected production macOS and Windows raster output plus the production preview SVG/CSS on light and dark backgrounds. Pixel inspection found shadow outside all four sides and fully transparent bitmap boundaries, with no clipped halo.
• Executed the production Windows rasterizer's extended pixel test in isolation on macOS — passed.
• node scripts/test-windows-computer-use-native.mjs --check --target x86_64-pc-windows-msvc — passed after the final test changes.
• pnpm --dir src/web-ui run test:run src/app/components/ComputerUseControlCard.test.tsx src/app/components/computer-use src/infrastructure/api/adapters/peer-device-adapter.test.ts — 79 passed.
• pnpm run check:web — passed, including type checking, Appearance contracts, and the color audit across all 26 registered surfaces.
• cargo test --locked -p openbitfun-desktop --features devtools --lib computer_use — 181 passed, 9 opt-in native tests ignored.
• cargo build --locked -p openbitfun-desktop --features devtools — passed.
• pnpm run fmt:rs, git diff --check, and the introduced Git object-size check — passed.

Reviewer Notes

AI-assisted, with focused automated testing and local visual verification. Windows native window rendering was not exercised on a Windows desktop; cross-compilation and production raster pixels were checked. Remote end-to-end scenarios were not exercised. No protocol or persisted-data changes.

Checklist

• This PR is focused and does not include secrets, temporary prompts, generated scratch files, or unrelated artifacts.
• Relevant verification is recorded above, or skipped checks are explained.
• User-facing strings, docs, and locales are updated where applicable (no string changes).