release-25.2: ui: move TimezoneProvider to be inside RequiredLogin

[kyle-a-wong]

Backport 1/1 commits from #169751 on behalf of @kyle-a-wong.

/cc @cockroachdb/release

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Fixes a bug where the login page was making a request for cluster settings in order to create the timezone provider context. This request results in a 401 unauthorized since users need to be authorized to request cluster setting data. In version before 26.3, this bug made it impossible to login as a non default tenant via OIDC login, since this unauthorized response caused tenant cookies to be cleared before user could click the OIDC button.

Epic: None
Release note: None

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Release justification: Fix for a bug that prevents non-default tenant OIDC login

[trunk-io]

😎 Merged successfully - details.

[blathers-crl]

Thanks for opening a backport.

Before merging, please confirm that it falls into one of the following categories (select one):

• Non-production code changes OR fixes for serious issues. Non-production includes test-only changes, build system changes, etc. Serious issues are defined in the policy as correctness, stability, or security issues, data corruption/loss, significant performance regressions, breaking working and widely used functionality, or an inability to detect and debug production issues.
• Other approved changes. These changes must be gated behind a disabled-by-default feature flag unless there is a strong justification not to. Reference the approved ENGREQ ticket in the PR body (e.g., "Fixes ENGREQ-123").

Add a brief release justification to the PR description explaining your selection.

Also, confirm that the change does not break backward compatibility and complies with all aspects of the backport policy.

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[cockroach-teamcity]

This change is 

[kyle-a-wong]

/trunk merge