chore: add Dependabot dependency update automation

[j-kon]

Summary

• add weekly Dependabot checks for the root Dart package and Flutter demo app
• monitor the native Cargo crate and GitHub Actions dependencies
• group minor and patch updates per ecosystem while keeping major upgrades separate

Closes #85

Validation

• parsed the YAML and verified all four ecosystem/directory scopes
• dart format --output=none --set-exit-if-changed lib test example bdk_demo/lib bdk_demo/test
• dart analyze --fatal-infos --fatal-warnings lib test example
• flutter analyze in bdk_demo
• dart test (22 passed, 5 integration tests skipped by their environment gate)
• flutter test (174 passed)