ci: add Dependabot updates

[artylobos]

Closes #85

Summary

• add .github/dependabot.yml for automated dependency updates
• cover the Cargo workspace at the repository root
• cover each npm package directory: sdk, cli, react, and indexer
• group ecosystem updates and cap open dependency PRs to keep update noise manageable

Validation

• git diff --check -- .github/dependabot.yml
• ruby -e 'require "yaml"; config = YAML.load_file(".github/dependabot.yml"); abort("wrong version") unless config["version"] == 2'
• Verified configured directories exist: /, /sdk, /cli, /react, /indexer

Bounty note

• Stellar Wave 5 — 200 Points