fix: restore eslint 9 and fix lint after dependency updates

[mattinannt]

Summary

• upgraded all dependencies to the newest version to solve dependabot issues.
• pin ESLint back to 9.39.2 in both packages for current ecosystem compatibility
• update playground lint script to run ESLint directly (Next.js 16 no longer supports the old lint CLI option)
• switch playground flat config to Next's flat exports and remove unused FlatCompat dependency
• refresh lockfile

Verification

• pnpm lint passes in all workspace packages

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[coderabbitai]

Walkthrough

The pull request modernizes the ESLint configuration in the playground app by replacing FlatCompat-based composition with direct Next.js preset imports, eliminating boilerplate code. It simultaneously updates dependencies across the monorepo: the playground app's runtime and development dependencies, root-level package manager and build tool versions, and the JavaScript package's testing and linting tools. These changes reflect a shift to a more straightforward ESLint configuration approach while maintaining current dependency versions.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately summarizes the main changes: fixing and restoring ESLint 9 compatibility and fixing linting after dependency updates.
Description check	✅ Passed	The description is directly related to the changeset, providing clear context about dependency upgrades, ESLint pinning, lint script updates, and configuration changes.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Merge Conflict Detection	✅ Passed	✅ No merge conflicts detected when merging into main

_{✏️ Tip: You can configure your own custom pre-merge checks in the settings.}

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