Add Dependabot configuration for npm and Cargo

[Ejirowebfi]

Description

There is no Dependabot configuration. Outdated dependencies (especially stellar-sdk and soroban-sdk) may contain security vulnerabilities.

Tasks

• Create .github/dependabot.yml with entries for npm (frontend) and cargo (contracts)
• Set weekly update schedule
• Group minor/patch updates into a single PR to reduce noise
• Add reviewers and assignees to the config

Acceptance Criteria

• Dependabot PRs are opened weekly for outdated dependencies
• Major version bumps are separate PRs from minor/patch
• Config file passes GitHub Dependabot schema validation