fix: Handle undefined signupSources gracefully

[sentry]

This PR addresses a TypeError: Cannot read properties of undefined (reading 'includes') that occurred when user.signupSources was undefined at runtime, despite being typed as a non-null array in the schema and types.

The root cause was that the optional chain user?. only guarded against user being nullish, not against signupSources itself being undefined (likely due to legacy user data or serialization issues).

The fix involves a two-pronged approach:

1. Client-side (src/components/NewsletterSignup.tsx): Added optional chaining (?.) to signupSources to prevent the client-side crash when calling .includes.
2. Server-side (src/auth/auth.server.ts, src/utils/users.server.ts): Implemented nullish coalescing (?? []) when reading signupSources from the database. This ensures that signupSources is always an array ([]) for all downstream consumers, preventing similar TypeErrors in other parts of the application and enforcing the contract at the data retrieval layer.

Fixes TANSTACK-COM-4FJ

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Status	Name	Latest Commit	Preview URL	Updated (UTC)
✅ Deployment successful! View logs	tanstack-com	b2237df	Commit Preview URL Branch Preview URL	Sep 15 2026, 11:05 AM