feat(settings): link the Steam account through Steam

A Steam card in the settings links, verifies or unlinks the Steam
account through "Sign in through Steam". After the way back the page
shows the outcome and refreshes the session, so Core gets the new
Steam ID at once. Registration and settings no longer ask for a typed
Steam ID.

src/pages/SettingsPage.steamToast.test.jsx

@@ -0,0 +1,56 @@
+// @vitest-environment jsdom
+import {afterEach, beforeEach, describe, expect, it, vi} from 'vitest';
+import {act, cleanup, render, screen} from '@testing-library/react';
+import {MemoryRouter} from 'react-router-dom';
+import {toast} from 'sonner';
+import SettingsPage from './SettingsPage.jsx';
+import {PreferencesProvider} from '../context/PreferencesContext.jsx';
+
+// Like react-i18next: a language change re-renders every subscriber with a fresh `t`.
+vi.mock('react-i18next', async () => {
+    const {useSyncExternalStore} = await import('react');
+    const listeners = new Set();
+    const state = {language: 'de', t: (key) => key};
+    const i18n = {
+        get language() { return state.language; },
+        changeLanguage: vi.fn((language) => {
+            state.language = language;
+            state.t = (key) => key;
+            listeners.forEach((listener) => listener());
+        }),
+    };
+    const subscribe = (listener) => { listeners.add(listener); return () => listeners.delete(listener); };
+    return {useTranslation: () => ({t: useSyncExternalStore(subscribe, () => state.t), i18n}), i18n};
+});
+vi.mock('sonner', () => ({toast: {success: vi.fn(), error: vi.fn(), info: vi.fn()}}));
+
+const profile = {
+    id: 1, email: 'a@b.de', role: 'USER', displayName: 'Lobo', steamId: '76561198005679788', steamIdVerified: true,
+    settings: {useDarkTheme: true, language: 'de', publicProfile: false},
+};
+
+describe('SettingsPage Steam toast', () => {
+    beforeEach(() => {
+        localStorage.setItem('tbp_session', '1');
+        globalThis.fetch = vi.fn(async (url, init) => (init?.method && init.method !== 'GET'
+            ? {ok: true, status: 204, json: async () => { throw new SyntaxError('no body'); }}
+            : {ok: true, status: 200, json: async () => profile}));
+    });
+
+    afterEach(() => {
+        cleanup();
+        localStorage.clear();
+        vi.clearAllMocks();
+    });
+
+    it('shows the linked toast once, also after a language change', async () => {
+        render(<MemoryRouter initialEntries={['/settings?steam=linked']}><PreferencesProvider><SettingsPage/></PreferencesProvider></MemoryRouter>);
+        await screen.findByText('SETTINGS.STEAM_LINK.VERIFIED');
+        expect(toast.success).toHaveBeenCalledTimes(1);
+
+        const {i18n} = await import('react-i18next');
+        await act(async () => { i18n.changeLanguage('en'); });
+
+        expect(toast.success).toHaveBeenCalledTimes(1);
+    });
+});