Add unit tests for the date/spreadsheet helpers and wire up CI

[devin-ai-integration]

Summary

The pure helpers in lib/ (event date math, CSV parsing, email/code extraction) had no coverage, and nothing ran on PRs. This adds Vitest plus a GitHub Actions job so lint, typecheck, and tests all run on every pull request.

Covered behavior worth calling out, since it's easy to regress:

• daysUntilEvent compares calendar days, not elapsed time — both sides are normalized to local midnight, so a 3pm now still yields 0 for an event today and 1 for tomorrow. Tests pass an explicit now (new Date(2026, 6, 17, 15, 30)) so they're deterministic, and one case spans a DST boundary where a naive (event - now) / 86400000 would be off.
• eventCountdownLabel maps 0 → "Today", 1 → "Tomorrow", n → "In n days", and null once past.
• extractCodes prefers a /^[\w\s]*codes?$/i header column but keeps every row of a headerless single-column file.

Runner is Node 22 in CI (Vitest 4 needs ≥20.19); npm test = vitest run.

Link to Devin session: https://app.devin.ai/sessions/a2b97d4e12ba4d19bedd24a6a58f237f
Requested by: @dabit3

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Project	Deployment	Actions	Updated (UTC)
vending-machine	Ready	Preview, Comment	Jul 28, 2026 11:33pm

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.