feat(webui): add Brazilian Portuguese (pt-BR) locale

Translate the WebUI common.json to pt-BR, register the locale in
config.ts (with normalizeLocale rules for "pt", "pt-BR", "pt-PT", ...)
and index.ts resources, and add a Brazilian Portuguese overview
assertion in i18n.test.tsx.

Notes:
- Generic "pt" and any "pt-*" variant resolve to pt-BR. A future
  pt-PT locale can be added without affecting existing users.
- Universally borrowed technical terms (nanobot, BYOK, MCP, JSON,
  URL, Web, Apps when used as a product name, Skills) are kept in
  English, matching the pattern used by es/fr/ja/ko/vi/id.
- Empty-thread greetings, quick-action prompts, and slash command
  descriptions are translated; quick-action prompts are intentionally
  phrased so the agent still receives a clear task spec.

Verified with bun run test (536 passing), bun run lint (clean), and
bun run build (7.18s).

webui/src/i18n/index.ts

@@ -19,6 +19,7 @@ import frCommon from "./locales/fr/common.json";
 import jaCommon from "./locales/ja/common.json";
 import koCommon from "./locales/ko/common.json";
 import esCommon from "./locales/es/common.json";
+import ptBRCommon from "./locales/pt-BR/common.json";
 import viCommon from "./locales/vi/common.json";
 import idCommon from "./locales/id/common.json";
 
@@ -30,6 +31,7 @@ export const resources = {
   ja: { common: jaCommon },
   ko: { common: koCommon },
   es: { common: esCommon },
+  "pt-BR": { common: ptBRCommon },
   vi: { common: viCommon },
   id: { common: idCommon },
 } as const;