Add Renovate configuration for automated dependency management

[kantacky]

Summary

This PR introduces a Renovate configuration file to enable automated dependency updates and maintenance for the project.

Key Changes

• Added renovate.json with recommended configuration preset
• Enabled dependency dashboard and semantic commit messages
• Configured weekly update schedule in Asia/Tokyo timezone
• Set up automated lock file maintenance on Monday mornings
• Configured auto-merge for dev dependency minor and patch updates
• Added special handling for Node.js version management via mise
• Grouped Next.js core packages (react, react-dom, next) for coordinated updates

Implementation Details

• Uses Renovate's recommended configuration as the base
• Groups all non-major updates together to reduce PR noise
• Dev dependencies are automatically merged when safe (minor/patch only)
• Lock file maintenance runs weekly before 6 AM on Mondays to keep dependencies fresh
• Next.js ecosystem packages are grouped to ensure compatibility between related packages
• All dependency PRs are labeled with "dependencies" for easy filtering

https://claude.ai/code/session_01QhBMBpjegevedg7p3VLjyR