fix: ESLint 10 CI lint failures

[Copilot]

Is your PR related to an issue? Please describe.
No.

Describe what problem you have solved or what feature you have added.
CI lint job was failing after bumping ESLint 9→10 due to three breaking changes:

• Node.js version (npm-lint.yml): ESLint 10 requires ^20.19.0 || ^22.13.0 || >=24; CI was pinned to Node 18. Updated to Node 20.
• Missing @eslint/js dep (package.json): ESLint 10 unbundled @eslint/js into a standalone package. Added it explicitly.
• no-useless-assignment error (extension.js:635): This rule was promoted into js.configs.recommended in ESLint 10. Fixed the useless initializer:

  // before
  let stateConf = false;  // immediately overwritten in both if/else branches
  
  // after
  let stateConf;

Additional context
All pre-existing no-shadow warnings are unrelated and unchanged.

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[Copilot]

Pull request overview

This PR fixes CI lint failures after upgrading to ESLint 10 by aligning the repo’s tooling dependencies and CI runtime with ESLint 10’s new requirements.

Changes:

• Add @eslint/js as an explicit dependency to satisfy ESLint 10’s unbundling.
• Update the GitHub Actions lint workflow to use Node 20 (instead of 18).
• Remove a useless assignment flagged by ESLint 10 (no-useless-assignment) in the GNOME extension code.

Reviewed changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated 1 comment.

File	Description
package.json	Adds @eslint/js dependency required by the existing flat config import.
package-lock.json	Locks @eslint/js and associated metadata for reproducible installs.
customize-ibus@hollowman.ml/extension.js	Removes an initializer that is immediately overwritten, satisfying ESLint 10’s recommended rules.
.github/workflows/npm-lint.yml	Bumps CI Node version to address ESLint 10 engine requirements.

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[Copilot]

node-version: 20 may still install a Node 20.x release below ESLint 10's required engine (^20.19.0 || ^22.13.0 || >=24 as recorded in the lockfile), which can make the workflow flaky depending on what version setup-node resolves. Consider pinning the workflow to a minimum compatible version (e.g. 20.19.0 or 22.13.0) to match ESLint's engine requirement deterministically.

Suggested change

	node-version: 20
	node-version: '20.19.0'