fix(i18n): initialize locale from browser/localStorage and add fallback chain

- Initialize useState with getInitialLocale() instead of hardcoded 'en',
  so the correct locale is used from the very first render
- Default to 'pt-BR' instead of 'en' for SSR and fallback
- Add fallback chain in t(): tries current locale -> pt-BR -> en
- Extract resolveKey() helper for cleaner key resolution
- Cast dictionaries as Record<string, unknown> to avoid type issues
- Wrap localStorage access in try-catch for blocked storage scenarios

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/lib/i18n.tsx

@@ -1,6 +1,6 @@
 'use client';
 
-import React, { createContext, useContext, useState, useCallback, ReactNode, useEffect, startTransition } from 'react';
+import React, { createContext, useContext, useState, useCallback, ReactNode, useEffect } from 'react';
 import en from '@/i18n/en.json';
 import es from '@/i18n/es.json';
 import ptBR from '@/i18n/pt-BR.json';
@@ -13,12 +13,14 @@ interface I18nContextType {
     t: (key: string, params?: Record<string, string | number>) => string;
 }
 
-const dictionaries: Record<Locale, typeof en> = {
-    en,
-    es,
-    'pt-BR': ptBR,
+const dictionaries: Record<Locale, Record<string, unknown>> = {
+    en: en as Record<string, unknown>,
+    es: es as Record<string, unknown>,
+    'pt-BR': ptBR as Record<string, unknown>,
 };
 
+const VALID_LOCALES: Locale[] = ['en', 'es', 'pt-BR'];
+
 const I18nContext = createContext<I18nContextType | null>(null);
 
 const localeStorageKey = 'locale';
@@ -30,63 +32,77 @@ const normalizeLocale = (language: string): Locale => {
 };
 
 const getInitialLocale = (): Locale => {
-    if (typeof window === 'undefined') return 'en';
-    const storedLocale = localStorage.getItem(localeStorageKey);
-    if (storedLocale && Object.keys(dictionaries).includes(storedLocale)) {
-        return storedLocale as Locale;
+    if (typeof window === 'undefined') return 'pt-BR';
+    try {
+        const storedLocale = localStorage.getItem(localeStorageKey);
+        if (storedLocale && VALID_LOCALES.includes(storedLocale as Locale)) {
+            return storedLocale as Locale;
+        }
+    } catch {
+        // localStorage might be blocked
     }
     if (typeof navigator !== 'undefined' && navigator.language) {
         return normalizeLocale(navigator.language);
     }
-    return 'en';
+    return 'pt-BR';
 };
 
+function resolveKey(dict: Record<string, unknown>, key: string): string | undefined {
+    const keys = key.split('.');
+    let value: unknown = dict;
+
+    for (const k of keys) {
+        if (value && typeof value === 'object' && value !== null && k in value) {
+            value = (value as Record<string, unknown>)[k];
+        } else {
+            return undefined;
+        }
+    }
+
+    return typeof value === 'string' ? value : undefined;
+}
+
 export function I18nProvider({ children }: { children: ReactNode }) {
-    const [locale, setLocaleState] = useState<Locale>('en');
+    const [locale, setLocaleState] = useState<Locale>(getInitialLocale);
 
     const setLocale = (newLocale: Locale) => {
         setLocaleState(newLocale);
     };
 
     const t = useCallback((key: string, params?: Record<string, string | number>) => {
-        const keys = key.split('.');
-        let value: unknown = dictionaries[locale];
+        // Try current locale first, then fallback to pt-BR, then en
+        const dict = dictionaries[locale] || dictionaries['pt-BR'];
+        let result = resolveKey(dict, key);
 
-        for (const k of keys) {
-            if (value && typeof value === 'object' && value !== null && k in value) {
-                value = (value as Record<string, unknown>)[k];
-            } else {
-                return key;
-            }
+        // Fallback chain: try pt-BR if not found, then en
+        if (result === undefined && locale !== 'pt-BR') {
+            result = resolveKey(dictionaries['pt-BR'], key);
+        }
+        if (result === undefined && locale !== 'en') {
+            result = resolveKey(dictionaries['en'], key);
         }
 
-        if (typeof value !== 'string') return key;
+        if (result === undefined) {
+            return key;
+        }
 
         if (params) {
             return Object.entries(params).reduce(
-                (str, [paramKey, paramValue]) => str.replace(`{${paramKey}}`, String(paramValue)),
-                value
+                (str, [paramKey, paramValue]) => str.replace(new RegExp(`\\{${paramKey}\\}`, 'g'), String(paramValue)),
+                result
             );
         }
 
-        return value;
+        return result;
     }, [locale]);
 
-    // Restore locale from localStorage after hydration completes.
-    // Using startTransition to mark this as non-urgent so React
-    // finishes hydration before re-rendering with the stored locale.
-    useEffect(() => {
-        const stored = getInitialLocale();
-        if (stored !== 'en') {
-            startTransition(() => {
-                setLocaleState(stored);
-            });
-        }
-    }, []);
-
     useEffect(() => {
         if (typeof window === 'undefined') return;
-        localStorage.setItem(localeStorageKey, locale);
+        try {
+            localStorage.setItem(localeStorageKey, locale);
+        } catch {
+            // localStorage might be blocked
+        }
         document.documentElement.lang = locale;
     }, [locale]);