Centralize deterministic demo data seeding

[Abidoyesimze]

Summary

Mock modules seed data independently, making demos and screenshots non-reproducible.

Affected files

• src/lib/mock-services.ts
• src/lib/seeded-rng.ts
• src/lib/mock-chart-data.ts

Tasks

• Use one seed strategy (via seeded-rng) across mock services and chart data.
• Document the default seed for contributors.
• Verify visual baselines remain stable or update intentionally.

Acceptance criteria

• Scope is limited to the files listed (or agreed alternatives in the PR).
• Change is verifiable: tests, screenshots, or written QA steps in the PR.
• No behavior regressions in related user flows.
• No new duplicate abstractions without a one-line rationale in the PR.

Suggested labels

cleanup, refactor, testing

Notes

Standalone cleanup issue from CLEANUP_ISSUES_2026-05.md. Complements the audit queue and product backlog.