fix(app): reject non-finite usd in amountForUsd

[Ayush7614]

amountForUsd(Infinity, 1, 18) returned the literal string "Infinity", which viem parseUnits rejects downstream in suggestFirstBuy.

Before: `if (!(usd > 0) || !(quoteUsd > 0) || !Number.isFinite(quoteUsd))` — Infinity passes `usd > 0`, divides to Infinity, toFixed(2) is "Infinity", trimZeros is a no-op, places=0 passes, returns "Infinity".

After: also require `Number.isFinite(usd)`, matching the quoteUsd guard. Adds regression coverage for Infinity/NaN on both inputs.

Verified locally on upstream/main base:

• npm run lint: pass
• npx tsc --noEmit -p .: pass
• full unit suite: 323 pass, 0 fail
• npm run build: pass

Summary by CodeRabbit

• Bug Fixes
  • Improved handling of invalid purchase amount inputs.
  • Non-finite values such as Infinity and NaN are now safely rejected instead of causing invalid amount conversion errors.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Advanced

Run ID: b5cf5311-5ee0-4538-9c2a-f17c5b1d71de

📥 Commits

Reviewing files that changed from the base of the PR and between d067240 and 74f6368.

📒 Files selected for processing (2)

• app/src/lib/launchpad/first-buy.test.ts
• app/src/lib/launchpad/first-buy.ts

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📝 Walkthrough

Walkthrough

amountForUsd now rejects non-finite USD inputs. Tests cover Infinity and NaN for both USD and quote values.

Changes

Amount validation

Layer / File(s)	Summary
Reject non-finite amount inputs app/src/lib/launchpad/first-buy.ts, app/src/lib/launchpad/first-buy.test.ts	amountForUsd returns null for non-finite USD or quote values. Tests cover Infinity and NaN inputs.

Priority: ⬇️ Low

Estimated code review effort: 1 (Trivial) | ~5 minutes

Change: Bug fix

Suggested reviewers: kevincodex1

Merge Risk: ⚪ Minimal · up to 74f63

Non-finite amount inputs are now rejected before formatting, preventing invalid purchase suggestions. No merge-blocking risk remains.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: rejecting non-finite usd inputs in amountForUsd.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 1 functions across 2 files.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

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[kevincodex1]

lgtm