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/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