Add Brazilian Portuguese (pt-BR) localization support

## Motivation

We are evaluating Buzz for adoption by Brazilian teams across desktop, mobile, and the new web experience. Conversations work well in Portuguese, but the English-only application chrome is an adoption barrier for non-technical users.

I would like to contribute Brazilian Portuguese (`pt-BR`) support and help maintain it as the interface evolves.

## Proposed direction

I suggest an incremental, reviewable rollout:

1. Add the localization runtime with English as the source and fallback locale.
2. Detect the system/browser locale, with a persisted language override in Settings.
3. Extract one bounded surface first: navigation, onboarding, Messages, and their accessibility labels.
4. Add a human-reviewed `pt-BR` catalogue with key/interpolation parity tests.
5. Continue surface by surface instead of a repository-wide translation sweep.

The locale should use the BCP 47 tag `pt-BR` and display as `Português (Brasil)`.

## Prior art and coordination

The original `block/buzz` repository already has localization discussion in block/buzz#2929 and overlapping implementation attempts in block/buzz#5087, block/buzz#5152, and block/buzz#4250. I do not want to create another incompatible localization architecture.

Before turning the proposal into a ready-for-review implementation, could the maintainers clarify:

- Is `block/buzz-app` now the preferred target for new desktop/web localization work?
- Is `react-i18next` acceptable here, or would a repository-local message catalogue be preferred?
- Is an incremental first PR covering the main shell and Messages the desired review size?

## Contribution commitment

I am willing to implement the infrastructure, provide the initial Brazilian Portuguese translation, test the complete user journey in Portuguese, and review terminology updates over time.

No files or tests are named. First clarify with maintainers whether buzz-app is the target, which localization architecture is preferred, and whether the proposed shell-and-Messages scope is appropriate; then inspect those surfaces and the existing locale setup. Done means an agreed incremental plan, runtime and fallback behavior, persisted locale selection, and reviewed pt-BR coverage with parity tests.

Written by the indexing model from the issue text.