test: cover transcript authorization errors

[Chris0Jeky]

Refs #1871

Summary

• Add coverage for both HTTP 401 and 403 transcript authorization failures using the real transport error shape.
• Assert the English unauthorized catalog value and the Spanish translation after a live locale switch.
• Restore the prior locale even if the test fails.

Verification

• TranscriptEvidenceViewer.spec.ts: 11/11
• Frontend typecheck: passed
• File-scoped ESLint: passed
• Mutation check: removing 403 from the production mapping made the HTTP 403 test receive the generic error and fail; production code was restored.
• git diff --check: passed

Two coordinator review passes completed for coverage correctness and scope/regression quality.

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