Establish visual regression baselines for deployment preview screenshots

[temma02]

Description

VisualRegressionTester in apps/backend/tests/visual/preview.visual.test.ts generates mock screenshots but no baselines are stored. Without baselines, the diff threshold check is never exercised and visual regressions go undetected.

Requirements and context

• Must be secure, tested, and documented where applicable
• Should stay reviewable and fit the current monorepo structure
• Relevant files: apps/backend/tests/visual/preview.visual.test.ts

Suggested execution

• Create branch: issue-015-visual-regression-baselines-deployment-preview
• Keep changes scoped to the issue and reference the task IDs in the PR

Implement changes

• Run storeBaseline for each template category (dex, defi, payment, asset) and commit the outputs
• Add a CI step that runs compareWithBaseline and fails on diff > threshold
• Document the baseline update workflow in a CONTRIBUTING.md section

Test and commit

• Confirm compareWithBaseline returns within threshold for unchanged templates
• Include before/after screenshots in the PR for each template category
• Edge case: baseline file missing — test should fail with a clear message, not silently pass

Example commit message

test(visual): store initial baselines for deployment preview

Guidelines

• Prefer small, reviewable PRs
• Keep naming and data contracts consistent with the spec docs

[Flamki]

@Flamki has applied to work on this issue as part of the Stellar Wave Program's 4th wave.

i would like to work on this issue

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[blockchainrafik]

@blockchainrafik has applied to work on this issue as part of the Stellar Wave Program's 4th wave.

Hi maintainer, i will like to work on this issue.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @blockchainrafik to this issue.

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This issue has been completed by @Flamki as part of the Stellar Wave Program's 4th Wave 🥳

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