Add Dependabot configuration for automated dependency updates

[Anuoluwapo25]

Task

Create .github/dependabot.yml so Dependabot automatically opens PRs for outdated dependencies.

Config to create

version: 2
updates:
  # npm (frontend)
  - package-ecosystem: "npm"
    directory: "/"
    schedule:
      interval: "weekly"
      day: "monday"
    labels: ["dependencies"]
    open-pull-requests-limit: 5

  # Cargo (smart contracts)
  - package-ecosystem: "cargo"
    directory: "/"
    schedule:
      interval: "weekly"
      day: "monday"
    labels: ["dependencies", "smart-contract"]
    open-pull-requests-limit: 3

  # GitHub Actions
  - package-ecosystem: "github-actions"
    directory: "/"
    schedule:
      interval: "weekly"
      day: "monday"
    labels: ["dependencies", "devops"]

Acceptance Criteria

• .github/dependabot.yml created with all 3 ecosystems
• No existing Dependabot config conflicts
• File is valid YAML (run python -c "import yaml; yaml.safe_load(open('.github/dependabot.yml'))" to verify)

Estimated time: 10 minutes