Fix registration update API crash: headers already sent

[irvingpop]

Summary

Fixes #1965

The PATCH /api/registration/update endpoint was crashing with "Cannot set headers after they are sent to the client" (Sentry FRONT-END-86, 9 occurrences).

Root cause

When a user completes the final registration step, the API clears the opCodeApplicantEmail cookie. But the multi-step form fires additional PATCH requests after that (e.g. rapid clicking). These arrive with no cookie, the Airtable lookup returns nothing, and the code hits a broken res.writeHead(404).json() path -- writeHead() flushes headers immediately, so the subsequent .json() (which sets Content-Type) throws. The catch block's res.status(500).json() fails for the same reason, producing the unhandled error.

Confirmed by tracing the actual Sentry events: the user had 4 successful 200 PATCHes (filling all fields, which cleared the cookie), followed by 7 consecutive 500s.

Changes

• pages/api/registration/update.ts: Replace writeHead(404).json() with status(404).json(). Add early 401 return when cookie is missing. Clear stale cookie on 404.
• components/Forms/UpdateProfileForm/UpdateProfileForm.tsx: Detect 404 from the update API and redirect to /join instead of showing a dead-end error.
• .gitignore: Add tsconfig.tsbuildinfo and .mcp.json.

Test plan

• Existing UpdateProfileForm snapshot test passes
• Lint and typecheck pass
• Verify: complete registration flow, confirm no errors on final step
• Confirm FRONT-END-86 stops recurring after deploy

🤖 Generated with Claude Code

[vercel]

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[github-actions]

Coverage Report

Status	Category	Percentage	Covered / Total
🔵	Lines	84.71%	5021 / 5927
🔵	Statements	84.71%	5021 / 5927
🔵	Functions	90.28%	158 / 175
🔵	Branches	93.53%	579 / 619

File Coverage

File	Stmts	Branches	Functions	Lines	Uncovered Lines
Changed Files
components/Forms/UpdateProfileForm/UpdateProfileForm.tsx	40%	50%	25%	40%	15-38, 60-94, 97-102, 106

Generated in workflow #47 for commit 2b5f981 by the Vitest Coverage Report Action

[kylemh]

dint review 🤷🏼‍♂️