chore: bump eslint to 10.3.0 + migrate config (drop eslint-plugin-header, switch to eslint-plugin-import-x)

[dependabot]

Summary

Bumps eslint from 9.39.4 to 10.3.0 (dependabot's original bump) and migrates the project's lint setup to be ESLint 10–compatible.

What ESLint 10 broke

ESLint 10 removes the legacy context.getSourceCode() API and tightens peer-dep handling. Two plugins this project used don't survive:

• eslint-plugin-header@3.1.1 — the entire plugin relies on context.getSourceCode(). No maintained release supports ESLint 10. Dropped, along with the Microsoft copyright-header rule. The rule can be reintroduced via a custom rule or a different plugin when one becomes available.
• eslint-plugin-import@2.32.0 — peer range stops at ESLint 9. Replaced with eslint-plugin-import-x (the actively-maintained fork; supports ESLint 8/9/10). The import/order rule becomes import-x/order.

Other dependency changes

• Add @eslint/js@^10.0.1 as an explicit devDep — under yarn 4 strict peer-dep handling it isn't hoisted from eslint's transitive graph anymore.
• Add eslint-import-resolver-typescript@^4.4.4 — required by importX.flatConfigs.typescript to resolve .ts/.tsx paths.
• Pin all three to current latest minor.patch (^10.0.1, ^4.4.4, ^4.16.2).

Config modernization

Adopts the defineConfig helper from eslint/config (ESLint 10's recommended setup):

// @ts-check

import js from "@eslint/js";
import { defineConfig } from "eslint/config";
import eslintConfigPrettier from "eslint-config-prettier";
import importX from "eslint-plugin-import-x";
import tseslint from "typescript-eslint";

export default defineConfig(
  { ignores: ["*.config.*", "dist/**"] },
  js.configs.recommended,
  tseslint.configs.recommended,
  importX.flatConfigs.recommended,
  importX.flatConfigs.typescript,
  eslintConfigPrettier,
  { /* custom rules */ },
);

Custom rules and parser options preserved as-is.

Verification (Node 24)

• yarn install --immutable clean
• yarn dedupe --check clean
• yarn lint clean
• yarn format clean
• yarn build clean
• yarn test → 14/14 playwright tests pass

Follow-ups

• The Microsoft copyright-header rule needs a replacement when one ships for ESLint 10. Tracked separately.

[github-actions]

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