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/routes/login.tsx

@@ -1,5 +1,6 @@
 import { createFileRoute, useNavigate } from "@tanstack/react-router";
 import { useEffect } from "react";
+import { useTranslation } from "react-i18next";
 import { useAuth } from "../hooks/useAuth";
 
 export const Route = createFileRoute("/login")({
@@ -7,6 +8,7 @@ export const Route = createFileRoute("/login")({
 });
 
 function LoginPage() {
+	const { t } = useTranslation();
 	const navigate = useNavigate();
 	const { data: auth, isLoading } = useAuth();
 
@@ -19,7 +21,7 @@ function LoginPage() {
 	if (isLoading) {
 		return (
 			<div className="min-h-screen bg-gray-50 flex items-center justify-center">
-				<p className="text-gray-500 text-sm">Loading...</p>
+				<p className="text-gray-500 text-sm">{t("actions.loading")}</p>
 			</div>
 		);
 	}
@@ -28,11 +30,11 @@ function LoginPage() {
 		<div className="min-h-screen bg-gray-50 flex items-center justify-center px-4">
 			<div className="w-full max-w-sm">
 				<h1 className="text-xl font-semibold text-gray-900 text-center mb-6">
-					Sign in to GearBox
+					{t("auth.signInToGearBox")}
 				</h1>
 				<div className="bg-white rounded-xl border border-gray-100 p-6 space-y-4">
 					<p className="text-sm text-gray-500 text-center">
-						You will be redirected to sign in with your account.
+						{t("auth.redirectDescription")}
 					</p>
 					<button
 						type="button"
@@ -41,7 +43,7 @@ function LoginPage() {
 						}}
 						className="w-full py-2 px-4 text-sm font-medium text-white bg-gray-700 hover:bg-gray-800 rounded-lg transition-colors"
 					>
-						Sign In
+						{t("auth.signIn")}
 					</button>
 				</div>
 			</div>