fix(billing): Use canonical PAYG allowances in public estimator (fixes #399)

[stroland02]

This PR resolves an issue where the public PAYG estimator was overstating the estimated monthly bill. Previously, the estimator did not account for the included investigation credits and telemetry allowances. By updating \Pricing.tsx\ to import and utilize the canonical billing models from @superlog/billing, the calculator now correctly factors in these allowances before computing overage costs. A regression test has also been added to ensure the pricing estimator logic remains sound. Fixes #399.

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Summary by cubic

The public PAYG estimator now uses canonical billing rates and allowances from @superlog/billing, so estimates no longer overstate monthly costs. It correctly accounts for included investigation credits and telemetry before charging overages.

• Bug Fixes
  • Replaced local PAYG constants with PLANS, PAYG_RATES, and PAYG_PROMOTION_CREDITS from @superlog/billing.
  • Compute billable investigations and telemetry overage after included credits/allowances.
  • Added a regression test to confirm the default state displays $0.00 within allowances.

^{Written for commit 49bd827. Summary will update on new commits.}

[superlog-app]

Observability review

All clear — no observability concerns found.

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[cubic-dev-ai]

No issues found across 2 files

_{Re-trigger cubic}