ci: add Dependabot configuration

[tedteddy]

Summary

This PR adds Dependabot configuration for automated dependency updates.

Features

npm Dependencies

• Weekly checks on Monday at 9:00 AM (Asia/Shanghai timezone)
• Groups minor and patch updates together to reduce PR noise
• Ignores major version updates by default (for stability)
• Limit of 10 open PRs at a time

GitHub Actions

• Weekly checks for action version updates
• Keeps CI workflows up to date

Configuration Details

• Commit message prefix: chore(deps) for npm, ci(deps) for actions
• Automatic labels: dependencies, github-actions
• Grouped updates to minimize maintenance burden

Benefits

• Automated security patches
• Keep dependencies up to date
• Reduce manual maintenance effort
• Clear visibility into available updates