[2026 DevOps] Add preview deployments for every pull request

[Nanle-code]

Objective

Let reviewers try UI changes in a browser without checking out the branch.

Target area

.github/workflows/deploy.yml

Context

UI-heavy PRs are reviewed from diffs and screenshots. Preview URLs speed up review and let visual and a11y checks run against a real deployment.

Acceptance criteria

• Each PR gets a preview URL posted as a comment and updated on push.
• Previews use testnet-only configuration, and mainnet writes are disabled.
• Previews are torn down when the PR closes.
• Fork PRs are handled safely (no secrets exposed to untrusted code).
• Automated tests cover the primary flow, at least one boundary case, and at least one failure case.
• User-facing documentation or developer guidance is updated, including compatibility, security, or migration notes where applicable.

Pull request requirements

• All continuous integration checks must pass before a pull request may be merged.
• The branch must be free of merge conflicts with the target branch at the time of merge.
• Do not request merge with failing workflows, skipped required checks, or unresolved conflicts.

[drips-wave]

@Nanle-code This issue could not be added to the Stellar Wave Program because the per-repo points budget would be exceeded (25000 / 25000 points already used). The label has been automatically removed.

The budget resets at the end of each Wave. You can review the remaining budget for your repos on the Orgs & Repos page.

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