Upgrade frontend tooling and adopt shared flat lint configs

[obenland]

This follow-up upgrades Node.js from 20 to 24, @wordpress/scripts from 27.9.0 to 35.0.0, and @wordpress/env from 9.3.0 to 11.15.0 so the repository can use the shared flat ESLint and Stylelint configurations from wporg-repo-tools.

Stacked on #775. This PR targets chore/phpcs-tooling to show only the frontend changes; retarget it to trunk after #775 merges. Do not merge this PR into the temporary base branch.

• Replace the legacy ESLint wrapper with flat configuration and explicit generated/dependency-directory exclusions; scope lint commands to plugin sources.
• Move Stylelint out of package.json into a shared-config wrapper using the CSS preset for PostCSS sources. Preserve the existing line-length and duplicate-selector exceptions, and track .stylelintrc as the entry point for both CLI and editor autodiscovery.
• Identify WordPress-provided imports as ESLint externals without adding runtime npm dependencies, and resolve newly enforced lint findings with unused catch-binding removal, a corrected JSDoc type, CSS keyword spelling, and a documented legacy clipping fallback.

Validation: clean npm ci; Composer validation; PHPCS across 83 files with no errors or warnings; CSS lint; JS lint with zero errors and ten existing React Hook warnings; production build with the same 139 output paths; and PHP 8.2 tests in an isolated wp-env environment (32 tests, 107 assertions). Browser behavior has not been manually tested.

[Copilot]

Copilot was unable to review this pull request because the user who requested the review has reached their quota limit.