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.js

@@ -15,13 +15,6 @@ const inviteCodeField = z
     .string()
     .min(1, 'VALIDATION.INVITE_CODE_REQUIRED');
 
-// A SteamID64 has 17 digits. Keep it a string: a JS number loses precision above 2^53.
-const steamIdField = z
-    .string()
-    .trim()
-    .min(1, 'VALIDATION.STEAM_ID_REQUIRED')
-    .regex(/^\d{17}$/, 'VALIDATION.STEAM_ID_INVALID');
-
 // Same rule as the Auth service: 3-24 letters, digits, space, _ . - and no space at the ends.
 // An empty field means "no display name" and is sent as null.
 const displayNameField = z
@@ -39,7 +32,6 @@ export const registrationSchema = z
         password: passwordField,
         passwordConfirmation: z.string().min(1, 'VALIDATION.PASSWORD_CONFIRMATION_REQUIRED'),
         inviteCode: inviteCodeField,
-        steamId: steamIdField,
         displayName: displayNameField,
     })
     .refine((d) => d.password === d.passwordConfirmation, {
@@ -50,6 +42,5 @@ export const registrationSchema = z
 export const settingsSchema = z.object({
     useDarkTheme: z.boolean(),
     language: z.enum(['en', 'de'], {message: 'VALIDATION.LANGUAGE_INVALID'}),
-    steamId: steamIdField,
     displayName: displayNameField,
 });