chore(eslint): migrate to eslint 10

[elzody]

Summary

On my setup, npm run lint was broken because our ESLint 8.57 config walks up the directory tree and finds server's eslint.config.js and switches to flat config mode. This causes ESLint to reject the --ext option. The current format is also end-of-life, and we needed to upgrade anyway to avoid long-standing dependabot conflicts and to use the new Vue 3 rules now that the migration has been completed in #6142.

The three commits in this PR do the following:

1. Upgrades the dependencies and migrates the configuration to the new format
2. Runs npm run lint:fix to automatically resolve any fixable violations
3. Fix the remaining ~140 violations manually (I say manually but not really because this PR is Claude-assisted)

Several things were carried over from the old config like the JSDoc rules. but some new ones were added to account for new rules that help avoid too much churn for what was supposed to be a targeted migration. They can be followed up in separate PRs eventually.

The lock file diff is huge but that's because it migrated from v2 to v3 (matching other Nextcloud apps) and it also includes a bunch of small version bumps, but they are reasonable.

The failing test at the moment is due to a pre-existing issue with the files_pdfviewer app and is not related to these changes.

If it wasn't clear already, this PR is AI-assisted (Claude Opus 5).

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Documentation (manuals or wiki) has been updated or is not required