feat(i18n): extract strings from navigation, dialogs, onboarding, settings, and login

- Add useTranslation() to TopNav, BottomTabBar, FabMenu, UserMenu
- Internationalize ConfirmDialog, AuthPromptModal, ExternalLinkDialog
- Extract all onboarding flow strings (Welcome, HobbyPicker, ItemBrowser, Review, Done)
- Internationalize settings page (weight unit, currency, API keys, import/export)
- Internationalize login page and root error boundary
- All dialogs in __root.tsx use t() for UI chrome

Phase 34, Plan 02 (core navigation and global UI)

src/client/components/AuthPromptModal.tsx

@@ -1,7 +1,9 @@
 import { Link } from "@tanstack/react-router";
+import { useTranslation } from "react-i18next";
 import { useUIStore } from "../stores/uiStore";
 
 export function AuthPromptModal() {
+	const { t } = useTranslation();
 	const showAuthPrompt = useUIStore((s) => s.showAuthPrompt);
 	const closeAuthPrompt = useUIStore((s) => s.closeAuthPrompt);
 
@@ -18,10 +20,10 @@ export function AuthPromptModal() {
 			/>
 			<div className="relative bg-white rounded-xl shadow-lg p-6 max-w-sm mx-4 w-full">
 				<h3 className="text-lg font-semibold text-gray-900 mb-2">
-					Join GearBox
+					{t("auth.joinGearBox")}
 				</h3>
 				<p className="text-sm text-gray-600 mb-6">
-					To manage your own collection, sign in or sign up.
+					{t("auth.signInDescription")}
 				</p>
 				<div className="flex flex-col gap-3">
 					<Link
@@ -29,14 +31,14 @@ export function AuthPromptModal() {
 						className="w-full text-center px-4 py-2.5 text-sm font-medium text-white bg-gray-700 hover:bg-gray-800 rounded-lg transition-colors"
 						onClick={closeAuthPrompt}
 					>
-						Sign in
+						{t("auth.signIn")}
 					</Link>
 					<Link
 						to="/login"
 						className="w-full text-center px-4 py-2.5 text-sm font-medium text-gray-700 bg-gray-100 hover:bg-gray-200 rounded-lg transition-colors"
 						onClick={closeAuthPrompt}
 					>
-						Create account
+						{t("auth.createAccount")}
 					</Link>
 				</div>
 			</div>