feat(toolbar): add Alt hold-to-select mode

[alexgorbatchev]

Summary

• add an Alt hold-to-select interaction that temporarily opens the toolbar, keeps manual activation sticky, and uses a crosshair cursor across page content while held
• add keyboard shortcut coverage for Alt activation, release behavior, and the injected cursor styling
• update the README fork notice to call out the maintained fork and the new Alt hold-to-select capability

Testing

• pnpm exec vitest run src/components/page-toolbar-css/tests/index.test.tsx --project unit