feat(auth): align AWS console text patterns

[keenwilson]

Problem

Update strings in the console credentials login flow and region selector to match AWS console patterns.

Solution

• Reformat region selector label to "Region (optional)"

• Update signin flow text to use "Otherwise" instead of "Or"

---

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[github-actions]

• This pull request modifies code in src/* but no tests were added/updated.
  • Confirm whether tests should be added or ensure the PR description explains why tests are not required.

[valerena]

You probably need to update the name of the PR to match the accepted format https://github.com/aws/aws-toolkit-vscode/blob/master/CONTRIBUTING.md#pull-request-title

[github-actions]

• This pull request modifies code in src/* but no tests were added/updated.
  • Confirm whether tests should be added or ensure the PR description explains why tests are not required.
• This pull request implements a feat or fix, so it must include a changelog entry (unless the fix is for an unreleased feature). Review the changelog guidelines.
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