Set default update repository URL to fabledsword Forgejo instance

Pre-configure https://git.fabledsword.com/bvandeusen/FabledApp so update
checks work out of the box on fresh installs. The URL can still be
overridden in Settings if needed.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/providers/settings_provider.dart

@@ -51,7 +51,8 @@ final forgejoRepoUrlProvider =
 class ForgejoRepoUrlNotifier extends StateNotifier<String?> {
   final SharedPreferences _prefs;
   ForgejoRepoUrlNotifier(this._prefs)
-      : super(_prefs.getString(_kForgejoRepoUrl));
+      : super(_prefs.getString(_kForgejoRepoUrl) ??
+            'https://git.fabledsword.com/bvandeusen/FabledApp');
 
   Future<void> setUrl(String url) async {
     final clean = url.endsWith('/') ? url.substring(0, url.length - 1) : url;