feat: wire early exit modal to live preview

[xx7412421-cloud]

Closes #579

Summary

• Fetch live early-exit preview figures when the commitment early-exit modal opens.
• Disable final confirmation while the preview is loading, then show live penalty/net refund values when available.
• Keep preview failures non-blocking by showing an inline warning and falling back to the local estimate.
• Add helper coverage for API-envelope parsing, 0% grace-period previews, and failed preview responses.

Validation

• npm exec -- vitest run tests/components/CommitmentEarlyExitModal.test.tsx tests/components/earlyExitPreview.test.ts
• npm exec -- eslint src/app/commitments/page.tsx src/components/CommitmentEarlyExitModal/CommitmentEarlyExitModal.tsx src/components/CommitmentEarlyExitModal/earlyExitPreview.ts tests/components/CommitmentEarlyExitModal.test.tsx tests/components/earlyExitPreview.test.ts
• npm exec -- prettier --check src/components/CommitmentEarlyExitModal/earlyExitPreview.ts tests/components/earlyExitPreview.test.ts
• git diff --check origin/master..HEAD

Full npm run test -- --run was also attempted, but the repository baseline still has unrelated failures outside this change, including duplicate declarations in backend modules, existing invalid JSX in CommitmentDetailsModal.tsx, missing @testing-library/user-event, jest is not defined in an existing Vitest file, and several tests missing DOM environment setup.

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[1nonlypiece]

the early-exit live-preview wiring looks like solid work, but the same process applies — #579 isn't assigned to you. please get assigned first, then reopen; i'm closing as invalid until then. ping us on https://discord.gg/WV7tdYkJk if you want help claiming it.