[Localization] Add Portuguese (Brazil) - pt-BR support

Edit: I noticed the i18n infrastructure is already implemented. I've finished the full translation to pt-BR and I'm providing the files below.

Hello @nextlevelbuilder,

I have completed the full localization of GoClaw to Portuguese (Brazil) - pt-BR.
This contribution includes:

• 39 translation modules in ui/web/src/i18n/locales/pt-BR/.
• Updated ui/web/src/i18n/index.ts to register the new locale and handle auto-detection.
• Updated ui/web/src/lib/constants.ts and en/topbar.json to include the language option in the UI selector.

The build has been tested and is working perfectly. I'm attaching a ZIP file with all the modified/new files to facilitate the integration.

Files included in the ZIP:

• ui/web/src/i18n/locales/pt-BR/ (New directory)
• ui/web/src/i18n/index.ts (Modified)
• ui/web/src/lib/constants.ts (Modified)
• ui/web/src/i18n/locales/en/topbar.json (Modified)

goclaw-ptbr-localization.zip

I hope this helps the community! Fixes #1112.

Inspect the attached files under ui/web/src/i18n/locales/pt-BR/ and compare the updates to ui/web/src/i18n/index.ts, ui/web/src/lib/constants.ts, and ui/web/src/i18n/locales/en/topbar.json. Run the UI build and verify that pt-BR is registered, auto-detection works, and the language appears in the selector with all 39 modules available.