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/onboarding/OnboardingHobbyPicker.tsx

@@ -1,4 +1,5 @@
 import { HOBBIES } from "@/shared/hobbyConfig";
+import { useTranslation } from "react-i18next";
 import { HobbyCard } from "./HobbyCard";
 
 interface OnboardingHobbyPickerProps {
@@ -12,14 +13,15 @@ export function OnboardingHobbyPicker({
 	onToggleHobby,
 	onContinue,
 }: OnboardingHobbyPickerProps) {
+	const { t } = useTranslation("onboarding");
 	return (
 		<div className="flex flex-col items-center justify-center min-h-screen px-8">
 			<div className="max-w-2xl text-center">
 				<h1 className="text-3xl font-bold text-gray-900 mb-2">
-					What are you into?
+					{t("hobby.title")}
 				</h1>
 				<p className="text-base text-gray-500 mb-8">
-					Pick one or more — we'll show you popular gear for each.
+					{t("hobby.subtitle")}
 				</p>
 				<div className="flex flex-wrap justify-center gap-4 mb-8">
 					{HOBBIES.map((hobby) => (
@@ -39,7 +41,7 @@ export function OnboardingHobbyPicker({
 					disabled={selectedHobbies.length === 0}
 					className="px-8 py-3 bg-gray-700 hover:bg-gray-800 disabled:opacity-50 disabled:cursor-not-allowed text-white font-medium rounded-lg transition-colors"
 				>
-					Continue
+					{t("hobby.continue")}
 				</button>
 			</div>
 		</div>