fix: repair IPFS deployment workflow

[kawmy]

Summary

• replace the nonexistent actions/checkout@v6 reference with actions/checkout@v4
• correct the Horizon build variable from PUBLIC_PUBLIC_STELLAR_HORIZON_URL to PUBLIC_STELLAR_HORIZON_URL

Closes #537

Verification

• repository pre-commit hook — passed
• focused workflow invariant check — passed
• YAML parsing — passed
• npx prettier .github/workflows/dapp-ipfs.yml --check — passed
• git diff --check — passed
• frontend lint — passed
• frontend production build — passed

Limitations

A live IPFS publication was not run locally because it requires the repository's dapp-ipfs environment variables, Storacha secrets, GitHub OIDC token, and attestation environment.

The base branch also has unrelated root-level validation failures: npm install encounters an existing ESLint peer-dependency conflict, and npm run build cannot find a root tsconfig.json. These pre-existing issues were not changed because #537 explicitly limits the scope to the checkout action and Horizon variable fixes.