chore: add Dependabot dependency update automation

[j-kon]

Summary

• add Dependabot version-update configuration
• monitor root Dart/pub dependencies
• monitor Flutter dependencies under bdk_demo
• monitor Rust dependencies under native
• monitor GitHub Actions used by repository workflows
• group minor and patch updates to reduce pull request noise
• run dependency checks weekly

Update policy

Minor and patch dependency updates are grouped by ecosystem.

Major dependency updates are intentionally left outside the groups so Dependabot can open them separately for focused review.

Scope

This PR only adds:

• .github/dependabot.yml

It does not:

• update dependency versions
• modify lockfiles
• change CI behavior
• enable automatic merging
• change native build or release logic

Validation

• confirmed valid Dependabot version 2 configuration structure
• confirmed package ecosystem directories match repository manifests
• confirmed CI passes
• confirmed the PR contains only the intended Dependabot configuration

Closes #85