test(recommendations): assert action-box financial range (closes #281)

[cristim]

Summary

Adds two regression tests in frontend/src/__tests__/recommendations.test.ts that pin the action-box summary's min/max range behaviour:

1. Multi-cell selection with a per-cell range: 2 cells, one with two term variants (1yr + 3yr), one single-variant. Asserts the summary renders $300 - $500/mo, $1,100 - $1,800 upfront, 2 cells.
2. Multi-cell selection with identical totals: when all per-cell sums collapse to the same min and max, formatSavingsRange returns a single value without the – separator. Asserts $400/mo and no – $400 substring.

Why no production change

The action-box render at frontend/src/recommendations.ts:3015-3023 already uses formatSavingsRange(min, max), which is exactly what the issue asks for. The "shows a single value instead of a range" symptom in the issue body predated the broader pageLevelRange + formatSavingsRange work that landed via the multi-cell-selection refactor. This PR just adds the missing test coverage so the range contract can't regress silently.

Locale note

formatCurrency calls toLocaleString(undefined, ...) which in Node/JSDOM may emit $1100 (no thousands separator) instead of $1,100. The regex makes the comma optional to stay robust across runtimes.

Test plan

• npx jest src/__tests__/recommendations.test.ts -t '#281' — both new tests pass
• Full npx jest src/__tests__/recommendations.test.ts no regressions

Closes #281

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• frontend/src/__tests__/recommendations.test.ts

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• Create PR with unit tests
• Commit unit tests in branch fix/281-wave2

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🧠 Learnings used

Learnt from: cristim
Repo: LeanerCloud/CUDly PR: 0
File: :0-0
Timestamp: 2026-05-05T15:44:54.463Z
Learning: In the LeanerCloud/CUDly repository, when a CodeRabbit review flags something as "outside-diff" that was already implemented in an earlier commit of the same PR, it is likely a stale-snapshot artifact where the review diff was generated against a pre-fix view. The inline comment convention `(CR pass on PR #<number> Finding N)` is used by the maintainer (cristim) to mark lines that were changed in direct response to a prior CodeRabbit review pass.

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Learnt from: cristim
Repo: LeanerCloud/CUDly PR: 277
File: frontend/src/__tests__/recommendations.test.ts:2472-2476
Timestamp: 2026-05-05T07:46:01.902Z
Learning: In the CUDly frontend (`frontend/src/recommendations.ts`), `effectiveSavingsPct` intentionally has NO plausibility guard (no null return or warning for reconstructed percentages above per-term ceilings). The maintainer explicitly rejected that approach in PR `#277`. The fix for inflated percentages (issue `#274`) is purely a data-path fix: plumbing the provider's canonical `on_demand_cost` through to the frontend so the denominator is not reconstructed from potentially misleading `monthly_cost + savings + amortized`. Tests that pin the reconstructed >80% behavior are intentional documentation of the pre-fix path, not bugs.

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