Sync text zoom preference through the account instead of local settings

Local user://settings.cfg meant zoom level didn't follow a student
across devices/browsers. Question bank and CCS text scale now live on
the user's study profile in Mongo (QuestionTextScale/CcsTextScale),
synced through a new PUT /api/profile/me/text-scale endpoint and the
existing preferences GET/cache already used for study preferences.

Both question_player.gd and ccs_service.gd read the synced value at
startup and react to AccountProfileService.profile_changed so a value
pulled down after login (or changed from another session) applies
without a restart. Backend validates the 0.85-1.35 range and updates
only the field(s) provided via a targeted $set, so it can't clobber
the other scale or the unrelated study-profile preferences.

api/QuestionsApi/Services/UserProfileStore.cs

@@ -50,6 +50,20 @@ public sealed class UserProfileStore
             new FindOneAndUpdateOptions<UserStudyProfile> { IsUpsert = true, ReturnDocument = ReturnDocument.After });
     }
 
+    public async Task<UserStudyProfile> UpdateTextScaleAsync(string userId, UpdateTextScaleRequest request)
+    {
+        var update = Builders<UserStudyProfile>.Update.Set(value => value.UpdatedUtc, DateTime.UtcNow);
+        if (request.QuestionTextScale is { } questionScale)
+            update = update.Set(value => value.QuestionTextScale, questionScale);
+        if (request.CcsTextScale is { } ccsScale)
+            update = update.Set(value => value.CcsTextScale, ccsScale);
+        update = update.SetOnInsert(value => value.UserId, userId);
+        return await _profiles.FindOneAndUpdateAsync(
+            value => value.UserId == userId,
+            update,
+            new FindOneAndUpdateOptions<UserStudyProfile> { IsUpsert = true, ReturnDocument = ReturnDocument.After });
+    }
+
     public Task<bool> IsDeletedAsync(string userId) =>
         _deletedUsers.Find(value => value.UserId == userId).AnyAsync();