feat(i18n): add language picker to settings and sync i18n with persisted preference

- Add language picker (English/Deutsch) to settings page using pill-toggle pattern
- Import useLanguage hook and i18n instance in settings
- Language change persists via updateSetting and calls i18n.changeLanguage
- Add useEffect in RootLayout to sync i18n language with DB setting on load
- Language labels use native names (English, Deutsch) for identification

Phase 34, Plan 04

src/client/routes/__root.tsx

@@ -7,7 +7,7 @@ import {
 	useNavigate,
 	useRouter,
 } from "@tanstack/react-router";
-import { useState } from "react";
+import { useEffect, useState } from "react";
 import { useTranslation } from "react-i18next";
 import { Toaster } from "sonner";
 import "../app.css";
@@ -23,6 +23,7 @@ import { OnboardingFlow } from "../components/onboarding/OnboardingFlow";
 import { TopNav } from "../components/TopNav";
 import { useAuth } from "../hooks/useAuth";
 import { useDeleteCandidate } from "../hooks/useCandidates";
+import { useLanguage } from "../hooks/useLanguage";
 import { useOnboardingComplete } from "../hooks/useSettings";
 import { useResolveThread, useThread } from "../hooks/useThreads";
 import { useUIStore } from "../stores/uiStore";
@@ -82,6 +83,15 @@ function RootLayout() {
 	const location = useLocation();
 	const { data: auth, isLoading: authLoading } = useAuth();
 	const isAuthenticated = !!auth?.user;
+	const language = useLanguage();
+	const { i18n } = useTranslation();
+
+	// Sync i18n language with persisted setting
+	useEffect(() => {
+		if (language && i18n.language !== language) {
+			i18n.changeLanguage(language);
+		}
+	}, [language, i18n]);
 
 	// Candidate delete state
 	const confirmDeleteCandidateId = useUIStore(