feat: add Uzbek language support

[macdiesel]

Description:

Adds the Uzbek (uz) locale to the i18n module. This mirrors the prior Vietnamese support change (#825) and touches the same three files:

• src/i18n/lib.js — adds uz to supportedLocales, the @formatjs/intl-pluralrules and @formatjs/intl-relativetimeformat locale-data imports for uz, and the messagesShape PropTypes entry.
• src/i18n/countries.js — registers the i18n-iso-countries uz locale so country names localize in Uzbek.
• src/i18n/lib.test.js — updates the "missing locales" warning count (16 → 17) and adds the uz assertion.

Why: Uzbek translations already exist for downstream MFEs in the openedx-translations pipeline, but frontend-platform does not yet recognize uz as an expected locale. As a result it logs a dev-mode Unexpected locale: uz warning, and plural-rules / relative-time formatting and localized country names fall back rather than using Uzbek data. The required FormatJS (@formatjs/.../locale-data/uz) and i18n-iso-countries (langs/uz.json) data already ship with the existing dependencies, so no dependency changes are needed.

Testing:

• npm run test for src/i18n passes locally (29/29).
• eslint passes on the changed files.

Merge checklist:

• Consider running your code modifications in the included example app within frontend-platform. This can be done by running npm start and opening http://local.openedx.io:8080.
• Consider testing your code modifications in another local micro-frontend using local aliases configured via the module.config.js file in frontend-build.
• Verify your commit title/body conforms to the conventional commits format (e.g., fix, feat) and is appropriate for your code change. Consider whether your code is a breaking change, and modify your commit accordingly.

Post merge:

• After the build finishes for the merged commit, verify the new release has been pushed to NPM.

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