fix: use getApiErrorMessage in use-settings and update CLAUDE.md docs

- Replace getApiErrorKey with getApiErrorMessage in use-settings.ts
  so error toasts show translated messages instead of raw i18n keys
- Update CLAUDE.md files to reflect the new t('section.key') pattern
  and remove outdated Proxy-related gotchas

frontend/src/CLAUDE.md

@@ -64,8 +64,8 @@ User interactions trigger mutations/queries via hooks, which communicate with th
 #### `lib/locales/` — Internationalization (i18n)
 - **Locale files** (`en-US/`, `pt-BR/`, `zh-CN/`, `zh-TW/`, `ja-JP/`): Translation strings organized by feature
 - **`i18n.ts`**: i18next configuration with language detection
-- **`use-translation.ts`**: Custom hook with Proxy-based `t.section.key` access pattern
-- **Pattern**: Components call `useTranslation()` hook; access strings via `t.common.save`, `t.notebooks.title`
+- **`use-translation.ts`**: Thin wrapper around react-i18next's `useTranslation` with language change events
+- **Pattern**: Components call `useTranslation()` hook; access strings via `t('common.save')`, `t('notebooks.title')`
 
 ## Data & Control Flow Walkthrough
 

frontend/src/lib/hooks/CLAUDE.md

@@ -10,7 +10,7 @@ React hooks for API data fetching, state management, and complex workflows (chat
 - **Streaming hooks** (`useAsk`): SSE parsing for multi-stage Ask workflows (strategy → answers → final answer)
 - **Model/config hooks** (`useModels`, `useSettings`, `useTransformations`): Application-level settings and model management
 - **Utility hooks** (`useMediaQuery`, `useToast`, `useNavigation`, `useAuth`): UI state and auth checking
-- **i18n hook** (`useTranslation`): Proxy-based translation access with `t.section.key` pattern and language switching
+- **i18n hook** (`useTranslation`): Thin wrapper around react-i18next with `t('section.key')` pattern and language switching
 
 ## Important Patterns
 
@@ -22,7 +22,7 @@ React hooks for API data fetching, state management, and complex workflows (chat
 - **SSE streaming pattern**: `useAsk` manually parses newline-delimited JSON from `/api/search/ask`; handles incomplete buffers
 - **Status polling**: `useSourceStatus` auto-refetches every 2s while `status === 'running' | 'queued' | 'new'`
 - **Context building**: `useNotebookChat.buildContext()` assembles selected sources + notes with token/char counts
-- **i18n Proxy pattern**: `useTranslation` returns `t` object with Proxy; access `t.section.key` instead of `t('section.key')`
+- **i18n pattern**: `useTranslation` returns standard react-i18next `t` function; access translations via `t('section.key')`
 
 ## Key Dependencies
 
@@ -49,8 +49,7 @@ React hooks for API data fetching, state management, and complex workflows (chat
 - **Status polling race**: `useSourceStatus` may refetch stale data before server catches up; retry logic has 3-attempt limit
 - **Keyboard trap in dialogs**: Some hooks manage modal state; ensure Dialog/Modal components handle escape key properly
 - **Form data handling**: `useFileUpload` and source creation convert JSON fields to strings in FormData
-- **useTranslation depth limit**: Proxy limits nesting to 4 levels; deeper access returns path string as fallback
-- **useTranslation loop detection**: >1000 accesses to same key in 1s triggers error and breaks recursion
+- **useTranslation**: Thin wrapper preserving `setLanguage` with language change events for `LanguageLoadingOverlay`
 
 ## Testing Patterns
 
@@ -187,7 +186,7 @@ function CredentialSettings() {
 ### Important Notes
 
 - **Toast notifications**: All mutations show success/error toasts automatically
-- **i18n integration**: Toast messages use translation keys from `t.apiKeys.*` and `t.common.*`
+- **i18n integration**: Toast messages use translation keys from `t('apiKeys.*')` and `t('common.*')`
 - **Error handling**: Uses `getApiErrorKey()` utility to extract error messages from API responses
 - **Local test results**: `useTestCredential` stores results in local state (not cached in TanStack Query)
 - **Migration feedback**: Migration hooks show different toasts based on migrated/skipped/error counts

frontend/src/lib/hooks/use-settings.ts

@@ -3,7 +3,7 @@ import { settingsApi } from '@/lib/api/settings'
 import { QUERY_KEYS } from '@/lib/api/query-client'
 import { useToast } from '@/lib/hooks/use-toast'
 import { useTranslation } from '@/lib/hooks/use-translation'
-import { getApiErrorKey } from '@/lib/utils/error-handler'
+import { getApiErrorMessage } from '@/lib/utils/error-handler'
 import { SettingsResponse } from '@/lib/types/api'
 
 export function useSettings() {
@@ -30,7 +30,7 @@ export function useUpdateSettings() {
     onError: (error: unknown) => {
       toast({
         title: t('common.error'),
-        description: getApiErrorKey(error, t('common.error')),
+        description: getApiErrorMessage(error, (key) => t(key), 'common.error'),
         variant: 'destructive',
       })
     },

frontend/src/lib/locales/CLAUDE.md

@@ -1,6 +1,6 @@
 # Locales Module (i18n)
 
-Internationalization system providing multi-language UI support using i18next with type-safe translation access.
+Internationalization system providing multi-language UI support using i18next with standard `t()` function calls.
 
 ## Architecture
 
@@ -9,7 +9,7 @@ lib/
 ├── i18n.ts              # i18next initialization and configuration
 ├── i18n-events.ts       # Language change event emitters
 ├── hooks/
-│   └── use-translation.ts  # Custom hook with Proxy-based API
+│   └── use-translation.ts  # Thin wrapper around react-i18next with language change events
 ├── utils/
 │   └── date-locale.ts   # date-fns locale mapping
 └── locales/
@@ -28,7 +28,7 @@ lib/
 - **`i18n.ts`**: i18next initialization with language detection (localStorage → browser)
 - **`i18n-events.ts`**: Event emitters for language change start/end (used by loading overlay)
 - **`locales/index.ts`**: Central registry exporting all locales and `LanguageCode` type
-- **`use-translation.ts`**: Custom hook providing `t` object with nested property access
+- **`use-translation.ts`**: Thin wrapper around react-i18next returning `{ t, i18n, language, setLanguage }`
 
 ## Translation Structure
 
@@ -67,24 +67,36 @@ import { useTranslation } from '@/lib/hooks/use-translation'
 function MyComponent() {
   const { t, language, setLanguage } = useTranslation()
 
-  // Nested property access (Proxy-based)
-  return <h1>{t.notebooks.title}</h1>
+  // Standard t() function call
+  return <h1>{t('notebooks.title')}</h1>
 
-  // With interpolation
-  return <p>{t.common.updated.replace('{time}', timeAgo)}</p>
+  // With string interpolation
+  return <p>{t('common.updated').replace('{time}', timeAgo)}</p>
 
   // Change language
   await setLanguage('zh-CN')
 }
 ```
 
+### Functions that accept t as a parameter
+
+Use `TFunction` from i18next:
+
+```typescript
+import type { TFunction } from 'i18next'
+
+const getNavigation = (t: TFunction) => [
+  { name: t('navigation.sources'), href: '/sources' },
+]
+```
+
 ## Important Patterns
 
-- **Proxy-based access**: `t.section.key` instead of `t('section.key')` for better DX
-- **Type safety**: `TranslationKeys` type derived from `enUS` locale
+- **Standard t() calls**: `t('section.key')` — standard react-i18next pattern
 - **Language persistence**: Saved to localStorage, auto-detected on load
 - **Fallback**: Falls back to `en-US` if key missing in current locale
 - **Date localization**: Use `getDateLocale(language)` from `utils/date-locale.ts`
+- **Language change events**: `setLanguage` emits start/end events for `LanguageLoadingOverlay`
 
 ## Key Dependencies
 
@@ -117,13 +129,10 @@ function MyComponent() {
 
 ## Important Quirks & Gotchas
 
-- **Proxy depth limit**: `useTranslation` limits nesting to 4 levels to prevent infinite loops
-- **Blocked properties**: React internals (`__proto__`, `$$typeof`, etc.) are blocked from Proxy traversal
-- **Loop detection**: Access counts reset every 1s; >1000 accesses triggers error and breaks recursion
-- **String methods**: `.replace()`, `.split()` work on translated strings via Proxy magic
 - **Language change events**: `emitLanguageChangeStart/End` used by `LanguageLoadingOverlay` for UX
 - **No SSR**: `useSuspense: false` disables React Suspense for i18next (avoids hydration issues)
 - **All keys required**: Missing keys in non-English locales fall back to English; keep locales in sync
+- **ErrorBoundary**: Uses raw `enUS` locale object directly (class component, can't use hooks)
 
 ## Testing Patterns
 
@@ -131,7 +140,7 @@ function MyComponent() {
 // Mock useTranslation in tests (see test/setup.ts)
 vi.mock('@/lib/hooks/use-translation', () => ({
   useTranslation: () => ({
-    t: enUS,  // Use English locale directly
+    t: (key: string) => key,  // Identity function returns the key
     language: 'en-US',
     setLanguage: vi.fn(),
   }),