fix(login): improve error classification (#11926)

Closes #11923

# Which Problems Are Solved

Client-side gRPC errors (e.g. NotFound, InvalidArgument,
PermissionDenied) were being surfaced as HTTP 500 Internal Server
Errors, causing false SRE alerts and poor UI feedback.

# How the Problems Are Solved

- New transport interceptor (`error-classification.ts`): Automatically
enriches every ConnectError with httpStatus and isUserError metadata via
a `ClassifiedConnectError` wrapper
- Route handler protection (`route.ts`, `flow-initiation.ts`): Catches
classified errors from `getAuthRequest` / `getSAMLRequest` and returns
correct HTTP status codes instead of 500
- Type safety: Replaced all magic number error.code === 9 checks with
typed Code.FailedPrecondition + instanceof ConnectError across
`oidc.ts`, `saml.ts`, `password.ts`, `zitadel.ts`
- Classification-aware logging (`session.ts`): Client/user errors log at
warn level, server errors at error level
- Observability (`otel.ts`): Spans now include error.is_user_error and
http.status_code attributes for alert filtering

# Additional Changes

Gitignore `next-env.d.ts`: This file is auto-generated by Next.js on
every next dev and next build invocation. The two modes write slightly
different import paths (.next/dev/types/ vs .next/types/), which causes
git diff --exit-code to fail in CI whenever a developer runs next dev
locally before committing. Since Next.js regenerates it automatically,
there's no need to track it

---------

Co-authored-by: Ramon <[email protected]>
Co-authored-by: Copilot <[email protected]>

.gitignore

@@ -107,3 +107,6 @@ CLAUDE.md
 
 docs_old
 apps/docs/package-lock.json
+
+# Auto-generated by Next.js (rewrites on every dev/build)
+next-env.d.ts