Add unit tests for the upload/date helpers and a CI workflow

[devin-ai-integration]

Summary

The pure helpers behind the two riskiest admin paths — parsing an uploaded attendee/code spreadsheet (lib/spreadsheet.ts) and rendering event countdowns (lib/event-date.ts) — had no coverage, so a regression there only shows up as "the upload silently dropped a row". This adds Vitest plus tests that pin the behavior those functions already have, and wires lint + typecheck + tests into GitHub Actions on every PR.

Coverage pins the edge cases that are easy to break in a refactor:

• parseCsv: quoted fields, "" escapes, CRLF, blank-row filtering
• extractEmails: email found in any column, Name <a@b.co> cells, lowercasing, dedupe
• extractCodes: header-named column vs. headerless single-column files (every row is data), trim/dedupe
• event-date: daysUntilEvent ignores time-of-day on both sides, eventCountdownLabel → Today / Tomorrow / In N days / null when past

npm test runs vitest run; no test env config is needed since these are all node-side pure functions.

Link to Devin session: https://app.devin.ai/sessions/a325715ce3164734964a55f90b84eed7
Requested by: @dabit3

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Project	Deployment	Actions	Updated (UTC)
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[devin-ai-integration]

✅ Devin Review: No Issues Found

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