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/utils/validation.test.js

@@ -1,36 +1,20 @@
 import {describe, expect, it} from 'vitest';
 import {registrationSchema, settingsSchema} from './validation.js';
 
-const settings = (steamId) => ({useDarkTheme: true, language: 'de', steamId});
+const settings = () => ({useDarkTheme: true, language: 'de'});
 
-describe('steamId validation', () => {
-    it('keeps a SteamID64 as the exact string', () => {
-        // Number('76561198005679788') is 76561198005679790
-        expect(settingsSchema.parse(settings('76561198005679788')).steamId).toBe('76561198005679788');
-    });
-
-    it('trims whitespace', () => {
-        expect(settingsSchema.parse(settings(' 76561198005679788 ')).steamId).toBe('76561198005679788');
-    });
-
-    it.each(['', '12345', '7656119800567978x', '765611980056797880'])('rejects %j', (steamId) => {
-        expect(settingsSchema.safeParse(settings(steamId)).success).toBe(false);
-    });
-
-    it('keeps the exact string on registration', () => {
-        const result = registrationSchema.parse({
-            email: 'a@b.de',
-            password: 'password1',
-            passwordConfirmation: 'password1',
-            inviteCode: 'code',
+describe('Steam ID', () => {
+    it('is no form field any more: it comes only from "Sign in through Steam"', () => {
+        expect(settingsSchema.parse({...settings(), steamId: '76561198005679788'})).not.toHaveProperty('steamId');
+        expect(registrationSchema.parse({
+            email: 'a@b.de', password: 'password1', passwordConfirmation: 'password1', inviteCode: 'code',
             steamId: '76561198005679788',
-        });
-        expect(result.steamId).toBe('76561198005679788');
+        })).not.toHaveProperty('steamId');
     });
 });
 
 describe('displayName validation', () => {
-    const withName = (displayName) => ({...settings('76561198005679788'), displayName});
+    const withName = (displayName) => ({...settings(), displayName});
 
     it('turns an empty field into null (no display name)', () => {
         expect(settingsSchema.parse(withName('  ')).displayName).toBeNull();