Fix BackfillUserProfileIdsAsync crash on fresh mobile DB

On fresh installs, the UserProfileId columns may not exist yet
(migrations only run during sync, not at app startup). Added a
column existence check before attempting the backfill SQL to
prevent SQLite Error 1: 'no such column: UserProfileId'.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: davidortinau

src/SentenceStudio.Shared/Data/UserProfileRepository.cs

@@ -79,6 +79,13 @@ public async Task<UserProfile> GetAsync()
 
     private static async Task BackfillUserProfileIdsAsync(ApplicationDbContext db)
     {
+        // Check if UserProfileId column exists before attempting backfill.
+        // On fresh installs, the column may not exist until MigrateAsync() runs.
+        var cols = await db.Database.SqlQueryRaw<string>(
+            "SELECT name FROM pragma_table_info('SkillProfile') WHERE name = 'UserProfileId'").ToListAsync();
+        if (cols.Count == 0)
+            return; // Column doesn't exist yet — skip backfill
+
         // Assign unowned SkillProfiles by matching Language → UserProfile.TargetLanguage
         await db.Database.ExecuteSqlRawAsync(@"
             UPDATE SkillProfile SET UserProfileId = (