feat: add Brazilian Portuguese locale

[Alcimerio]

Linked issue

Closes #1639

Type of change

• Bug fix
• New feature (with prior issue + design alignment)
• Refactor / code improvement
• Docs

What does this PR do

Adds Brazilian Portuguese (pt-BR) as a supported client UI locale. System locales pt-BR and bare pt now resolve to Brazilian Portuguese, while non-Brazilian Portuguese regional locales such as pt-PT and pt-AO continue to fall back to English until dedicated resources exist.

How did you verify it

Manual web test:

• Switched the app to Brazilian Portuguese and checked the main home screen.

• Opened Settings → General and checked the language menu with Português brasileiro - Brazilian Portuguese.

Not verified:

• Desktop: I tried npm run dev:desktop, but Electron would not launch in my local checkout. Likely skill issue/local setup on my side.
• iOS/Android: not tested because I did not have simulator/device access at the time.

Checklist

• One focused change. Unrelated cleanups split out.
• npm run typecheck passes
• npm run lint passes
• npm run format ran (Biome)
• UI changes include screenshots or video for every affected platform tested
• Tests added or updated where it made sense

[greptile-apps]

Greptile Summary

This PR adds Brazilian Portuguese (pt-BR) as a fully-supported client UI locale, covering all migrated translation surfaces with complete key parity to English.

• New packages/app/src/i18n/resources/pt-BR.ts provides ~1,900 translated strings matching the English key shape exactly; the TypeScript type constraint (TranslationResources) and the key-parity test enforce this.
• locales.ts registers pt-BR in the SupportedLocale type, LANGUAGE_OPTIONS, native-name maps, and resolveSupportedLocale; the resolver explicitly maps only \"pt\" (bare) and \"pt-br\" to pt-BR, leaving pt-PT/pt-AO and other regional variants falling back to English — fully tested and documented.
• resources.test.ts adds ptBR to the key-parity, fallback-ratio (≤ 25% English strings), and interpolation-placeholder guards, and also fills in the pre-existing zhCN gap in the fallback-ratio test.

Confidence Score: 5/5

Safe to merge — adds a new locale with no changes to existing logic paths.

The change is additive: a new resource file, new entries in existing maps, and new test assertions. The TypeScript type system enforces key parity at compile time, the runtime key-parity and fallback-ratio tests guard translation completeness, and the locale-resolution tests explicitly cover the pt-PT/pt-AO exclusion. All existing locale paths are untouched.

No files require special attention.

Important Files Changed

Filename	Overview
packages/app/src/i18n/resources/pt-BR.ts	New complete pt-BR translation resource with full key parity to English; all interpolation placeholders preserved.
packages/app/src/i18n/locales.ts	Adds pt-BR to SupportedLocale, LANGUAGE_OPTIONS, LANGUAGE_NATIVE_NAMES, and LANGUAGE_NAMES_BY_LOCALE; resolveSupportedLocale explicitly maps only "pt" and "pt-br" to pt-BR, leaving pt-PT/pt-AO as English fallbacks.
packages/app/src/i18n/locales.test.ts	Good test coverage: explicit assertions for pt-BR explicit selection, system locale mapping (pt and pt-BR), and negative cases (pt-PT and pt-AO return "en").
packages/app/src/i18n/resources.test.ts	ptBR added to key-parity, fallback-ratio (zhCN also now included closing the prior gap), and interpolation-placeholder guards; model count label pinned.
packages/app/src/i18n/i18next.ts	ptBR resource registered under "pt-BR" key; straightforward and correct.
docs/i18n.md	pt-BR added to locale list; parity-test description updated; Batch 5A progress note added.
packages/app/src/screens/settings-screen.tsx	No logic changes; pt-BR flows through the LANGUAGE_OPTIONS import automatically.

_{Reviews (2): Last reviewed commit: "Test zh-CN translation fallback coverage" | Re-trigger Greptile}