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/TopNav.tsx

@@ -1,5 +1,6 @@
 import { Link, useMatchRoute, useNavigate } from "@tanstack/react-router";
 import { useState } from "react";
+import { useTranslation } from "react-i18next";
 import { useAuth } from "../hooks/useAuth";
 import { LucideIcon } from "../lib/iconData";
 import { useUIStore } from "../stores/uiStore";
@@ -42,6 +43,7 @@ function NavLinkOrButton({
 }
 
 export function TopNav() {
+	const { t } = useTranslation();
 	const { data: auth } = useAuth();
 	const isAuthenticated = !!auth?.user;
 	const openAuthPrompt = useUIStore((s) => s.openAuthPrompt);
@@ -82,7 +84,7 @@ export function TopNav() {
 							isAuthenticated={isAuthenticated}
 							onAuthPrompt={openAuthPrompt}
 						>
-							Home
+							{t("nav.home")}
 						</NavLinkOrButton>
 						<NavLinkOrButton
 							to="/collection"
@@ -91,7 +93,7 @@ export function TopNav() {
 							isAuthenticated={isAuthenticated}
 							onAuthPrompt={openAuthPrompt}
 						>
-							Collection
+							{t("nav.collection")}
 						</NavLinkOrButton>
 						<NavLinkOrButton
 							to="/setups"
@@ -100,7 +102,7 @@ export function TopNav() {
 							isAuthenticated={isAuthenticated}
 							onAuthPrompt={openAuthPrompt}
 						>
-							Setups
+							{t("nav.setups")}
 						</NavLinkOrButton>
 					</nav>
 
@@ -124,7 +126,7 @@ export function TopNav() {
 								onKeyDown={(e) => {
 									if (e.key === "Enter") handleSearch();
 								}}
-								placeholder="Search catalog..."
+								placeholder={t("nav.searchPlaceholder")}
 								className="bg-gray-50 border border-gray-200 rounded-lg pl-9 pr-3 py-2 text-sm text-gray-900 placeholder-gray-400 focus:outline-none focus:ring-2 focus:ring-gray-200 focus:border-gray-300 w-48 lg:w-64 transition-colors"
 							/>
 						</div>
@@ -137,7 +139,7 @@ export function TopNav() {
 								to="/login"
 								className="text-xs text-gray-500 hover:text-gray-700 transition-colors"
 							>
-								Sign in
+								{t("auth.signIn")}
 							</Link>
 						)}
 					</div>