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.Tests/UserProfileModelsTests.cs

@@ -0,0 +1,22 @@
+using QuestionsApi.Models;
+using Xunit;
+
+namespace QuestionsApi.Tests;
+
+public sealed class UserProfileModelsTests
+{
+    [Theory]
+    [InlineData(null, null, true)]
+    [InlineData(1.0, 1.0, true)]
+    [InlineData(0.85, 1.35, true)]
+    [InlineData(0.84, null, false)]
+    [InlineData(null, 1.36, false)]
+    public void UpdateTextScaleRequestValidator_EnforcesRange(double? questionScale, double? ccsScale, bool expectedValid)
+    {
+        var request = new UpdateTextScaleRequest { QuestionTextScale = questionScale, CcsTextScale = ccsScale };
+
+        var error = UpdateTextScaleRequestValidator.Validate(request);
+
+        Assert.Equal(expectedValid, error is null);
+    }
+}

api/QuestionsApi/Controllers/ProfileController.cs

@@ -27,6 +27,14 @@ public sealed class ProfileController(UserProfileStore profiles, IConfiguration
         return Ok(await profiles.UpsertAsync(UserId(), request));
     }
 
+    [HttpPut("me/text-scale")]
+    [Authorize(Roles = "USER,SUPER")]
+    public async Task<IActionResult> UpdateTextScale(UpdateTextScaleRequest request)
+    {
+        if (UpdateTextScaleRequestValidator.Validate(request) is { } error) return BadRequest(error);
+        return Ok(await profiles.UpdateTextScaleAsync(UserId(), request));
+    }
+
     [HttpGet("me/export")]
     [Authorize(Roles = "USER,SUPER")]
     public async Task<IActionResult> Export() => Ok(new

api/QuestionsApi/Models/UserProfileModels.cs

@@ -13,6 +13,8 @@ public sealed class UserStudyProfile
     public string PreparationStage { get; set; } = "pre_dedicated";
     public int WeeklyGoalHours { get; set; } = 10;
     public string PreferredStudyMode { get; set; } = "mixed";
+    public double QuestionTextScale { get; set; } = 1.0;
+    public double CcsTextScale { get; set; } = 1.0;
     public int Revision { get; set; }
     public DateTime UpdatedUtc { get; set; } = DateTime.UtcNow;
 }
@@ -25,6 +27,12 @@ public sealed class UpdateUserStudyProfileRequest
     public string PreferredStudyMode { get; set; } = "mixed";
 }
 
+public sealed class UpdateTextScaleRequest
+{
+    public double? QuestionTextScale { get; set; }
+    public double? CcsTextScale { get; set; }
+}
+
 public sealed class DeletedUserTombstone
 {
     [BsonId]
@@ -48,3 +56,18 @@ public static class UserStudyProfileValidator
         return null;
     }
 }
+
+public static class UpdateTextScaleRequestValidator
+{
+    private const double Min = 0.85;
+    private const double Max = 1.35;
+
+    public static string? Validate(UpdateTextScaleRequest request)
+    {
+        if (request.QuestionTextScale is { } questionScale && (questionScale < Min || questionScale > Max))
+            return "Question text scale must be between 0.85 and 1.35.";
+        if (request.CcsTextScale is { } ccsScale && (ccsScale < Min || ccsScale > Max))
+            return "CCS text scale must be between 0.85 and 1.35.";
+        return null;
+    }
+}

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();