Create-account links for Last.fm and ListenBrainz, and Link Spotify for ListenBrainz

ListenBrainz accounts are MusicBrainz accounts, so its sign-up link goes to MusicBrainz.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

apps/mobile/src/lib/connect.ts

@@ -1,6 +1,6 @@
 import * as Linking from 'expo-linking';
 import * as WebBrowser from 'expo-web-browser';
-import { SPOTIFY_LINK_PAGES } from '@musicshare/shared';
+import { SIGNUP_PAGES, SPOTIFY_LINK_PAGES } from '@musicshare/shared';
 import { api } from './api';
 
 export type SignInResult = 'connected' | 'cancelled';
@@ -25,3 +25,6 @@ export async function signInWithService(provider: 'plex' | 'lastfm'): Promise<Si
 /** Where users approve "Spotify Scrobbling" (Last.fm) or the Spotify link (ListenBrainz). */
 export const openSpotifyLinkPage = (on: keyof typeof SPOTIFY_LINK_PAGES = 'lastfm') =>
   WebBrowser.openBrowserAsync(SPOTIFY_LINK_PAGES[on]);
+
+/** Their sign-up page, in the in-app browser (ListenBrainz accounts start on MusicBrainz). */
+export const openSignupPage = (service: keyof typeof SIGNUP_PAGES) => WebBrowser.openBrowserAsync(SIGNUP_PAGES[service]);