Fix: GitHub OAuth: use === instead of = in user email matching (#3877)

[Nixxx19]

Fixes #3877

Changes

• File: server/config/passport.js (line 180)
• Fix: In the .find() callback when resolving which existing user to link for multiple accounts with the same GitHub emails, changed (u) => (u.email = primaryEmail) to (u) => (u.email === primaryEmail).
• Impact: .find() now correctly returns the user whose email matches primaryEmail instead of the first user, and no longer mutates u.email during iteration, preventing wrong account linking and silent corruption of other users' email fields.

Verification

I have verified that this pull request:

• has no linting errors (npm run lint)
• has no test errors (npm run test)
• has no typecheck errors (npm run typecheck)
• is from a uniquely-named feature branch and is up to date with the develop branch.
• is descriptively named and links to an issue number, i.e. Fixes #3877
• meets the standards outlined in the accessibility guidelines