Fix mouse coordinates under CSS scale transforms

[chen-985211]

Fixes #6023

Summary

• derive CSS scale factors from the transformed and layout dimensions
• invert positive axis-aligned scaling before applying terminal padding
• add unit coverage for scale down, scale up, non-uniform scale, padding, and zero-size fallback
• add a browser regression test for mouse tracking under an ancestor scale transform

Tests

• npm run build && npm run esbuild
• npm run test-unit -- out-esbuild/browser/input/Mouse.test.js
• npm run esbuild-demo-client && npm run esbuild-demo-server
• npm run test-integration -- --suite=core --project=Chromium test/playwright/MouseTracking.test.ts
• npm run lint-changes