fix(i18n): base CMS hooks on override strings

[kleberbaum]

Summary

• ensure the Jaen locale descriptor always exposes the raw strings map alongside flattened messages
• switch the CMS hooks and pages to source copy from the overridable strings payload so app-level overrides propagate everywhere

Testing

• yarn tsc -p packages/gatsby-plugin-jaen/tsconfig.json (fails: package missing from lockfile in workspace)

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Codex Task