build(deps): bump @hashgraph/sdk from 2.75.0 to 2.78.0 in /dapp-example

Bumps [@hashgraph/sdk](https://github.com/hiero-ledger/hiero-sdk-js) from 2.75.0 to 2.78.0.
- [Release notes](https://github.com/hiero-ledger/hiero-sdk-js/releases)
- [Changelog](https://github.com/hiero-ledger/hiero-sdk-js/blob/main/CHANGELOG.md)
- [Commits](hiero-ledger/hiero-sdk-js@v2.75.0...v2.78.0)

---
updated-dependencies:
- dependency-name: "@hashgraph/sdk"
  dependency-version: 2.78.0
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]