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);
+    }
+}