Migrate vue-app test project ESLint config to ESLint 10 flat config

[Copilot]

• Analyze CI test failures
• Identify root cause: @vue/cli-plugin-eslint@~5.0.0 only supports eslint@^6||^7||^8||^9, so bumping to ^10.1.0 caused ERESOLVE peer dep conflict
• Migrate vue-app eslint legacy config (eslintConfig in package.json) to flat config (eslint.config.mjs)
• Revert eslint back to ^9.17.0 (ESLint 9 supports flat config, and is the max feasible version for this project)
• Add @eslint/js@^9.17.0 explicitly (needed for import js from '@eslint/js' in the flat config)
• Remove legacy eslintConfig section from vue-app/package.json

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[akurtakov]

@copilot investigate test failures

[Copilot]

@copilot investigate test failures

The failure was caused by a peer dependency conflict: @vue/cli-plugin-eslint@~5.0.0 only supports eslint@^6||^7||^8||^9, so bumping eslint to ^10.1.0 caused an ERESOLVE error during npm install in the TestVue test setup.

Fixed in commit 5f97b90 by:

• Reverting eslint back to ^9.17.0 in vue-app/package.json (ESLint 9 supports flat config — the PR goal "where feasible")
• Adding @eslint/js@^9.17.0 as an explicit devDependency (needed for the import js from '@eslint/js' in the new eslint.config.mjs)