fix(developers): distinguish API errors from taken usernames in availability check

CheckUsernameAsync returned false (taken) on any API failure, making every
username appear taken when SilverDESK was unreachable. Now returns nullable
bool so errors show a warning instead of blocking submission.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

BlazorApp/Services/DeveloperApplicationService.cs

@@ -17,21 +17,30 @@ public class DeveloperApplicationService
         _logger = logger;
     }
 
-    public async Task<bool> CheckUsernameAsync(string username)
+    /// <summary>
+    /// Checks username availability. Returns: true = available, false = taken, null = error/unknown.
+    /// </summary>
+    public async Task<bool?> CheckUsernameAsync(string username)
     {
         try
         {
             var response = await _httpClient.GetAsync($"/api/auth/check-username/{Uri.EscapeDataString(username)}");
             if (!response.IsSuccessStatusCode)
-                return false;
+            {
+                _logger.LogWarning("Username check returned {StatusCode} for {Username}", response.StatusCode, username);
+                return null;
+            }
 
             var result = await response.Content.ReadFromJsonAsync<JsonElement>();
-            return result.TryGetProperty("available", out var available) && available.GetBoolean();
+            if (result.TryGetProperty("available", out var available))
+                return available.GetBoolean();
+
+            return null;
         }
         catch (Exception ex)
         {
             _logger.LogError(ex, "Error checking username availability for {Username}", username);
-            return false;
+            return null;
         }
     }