fix(app): encode server credentials as UTF-8 (origin #46225)

[kvnloo]

Summary

Refreshes the UTF-8 Basic-auth credential fix onto current dev.

authTokenFromCredentials now encodes username:password as UTF-8 bytes before standard Base64. ASCII output stays unchanged; regression coverage includes Latin-1-range and multibyte credentials.

Credit / provenance

This carries forward upstream anomalyco#46225 by @sdivyanshu90. Credit for the original diagnosis and implementation direction belongs to them. This downstream branch only refreshes that work onto the current tree and preserves focused regression coverage.

Current refresh

• rebased cleanly onto current dev on 2026-10-03
• 0 commits behind
• only packages/app/src/utils/server.ts and server.test.ts changed
• no unrelated dependency/lockfile changes

Verification

The original upstream PR reported its focused app tests and typecheck green. Current upstream CI should revalidate this refreshed head before merge.

github_writes_origin=0 — ready to promote when upstream write access is available.