test(visual): establish deployment preview screenshot baselines

[Flamki]

Summary

This PR establishes real visual regression baselines for deployment preview screenshots and wires them into CI.

• Replaced in-memory preview snapshots with file-backed baselines for dex, defi, payment, and �sset categories.
• Committed initial baseline artifacts under �pps/backend/tests/visual/baselines/deployment-preview.
• Added a CI workflow that runs baseline comparison and fails when diff exceeds the configured threshold.
• Documented the baseline compare/update workflow in CONTRIBUTING.md.
• Added explicit missing-baseline test coverage with a clear failure message.

Validation

• VISUAL_BASELINE_MODE=store npm run --workspace @craft/backend test -- tests/visual/preview.visual.test.ts

pm run --workspace @craft/backend test -- tests/visual/preview.visual.test.ts

Issue

Fixes #489

Follow-up

• Attach before/after screenshots for each template category: dex, defi, payment, �sset.

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