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/Components/Pages/Developers.razor

@@ -107,6 +107,10 @@
                             {
                                 <span class="username-status username-taken">&#10007; Username is already taken</span>
                             }
+                            else if (_usernameCheckState == UsernameCheckState.Error)
+                            {
+                                <span class="username-status username-error">&#9888; Could not check availability — you can still submit</span>
+                            }
                             <ValidationMessage For="() => _application.DesiredUsername" />
                         </div>
 
@@ -209,7 +213,7 @@
                     {
                         <div class="dev-error">@_errorMessage</div>
                     }
-                    <button type="submit" class="dev-btn dev-btn-primary" disabled="@_submitting">
+                    <button type="submit" class="dev-btn dev-btn-primary" disabled="@(_submitting || _usernameCheckState == UsernameCheckState.Taken)">
                         @if (_submitting)
                         {
                             <span class="btn-spinner"></span>
@@ -240,7 +244,7 @@
 
     private readonly string[] _availablePlatforms = { "Windows", "macOS", "Linux", "Android", "iOS", "Other" };
 
-    private enum UsernameCheckState { None, Checking, Available, Taken }
+    private enum UsernameCheckState { None, Checking, Available, Taken, Error }
 
     private void SelectRole(ApplicationRole role)
     {
@@ -279,7 +283,12 @@
 
             if (!token.IsCancellationRequested)
             {
-                _usernameCheckState = available ? UsernameCheckState.Available : UsernameCheckState.Taken;
+                _usernameCheckState = available switch
+                {
+                    true => UsernameCheckState.Available,
+                    false => UsernameCheckState.Taken,
+                    null => UsernameCheckState.Error
+                };
                 StateHasChanged();
             }
         }

BlazorApp/Endpoints/DeveloperEndpoints.cs

@@ -12,7 +12,9 @@ public static class DeveloperEndpoints
         group.MapGet("/check-username/{username}", async (string username, DeveloperApplicationService service) =>
         {
             var available = await service.CheckUsernameAsync(username);
-            return Results.Ok(new { available });
+            if (available is null)
+                return Results.Problem("Unable to verify username availability", statusCode: 503);
+            return Results.Ok(new { available = available.Value });
         });
 
         group.MapPost("/apply", async (DeveloperApplication application, DeveloperApplicationService service) =>

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;
         }
     }
 

BlazorApp/wwwroot/developers-styles.css

@@ -186,6 +186,29 @@
     margin-top: 0.3rem;
 }
 
+/* Username status */
+.username-status {
+    display: block;
+    font-size: 0.82rem;
+    margin-top: 0.35rem;
+}
+
+.username-checking {
+    color: rgba(255, 255, 255, 0.5);
+}
+
+.username-available {
+    color: #34d399;
+}
+
+.username-taken {
+    color: #f87171;
+}
+
+.username-error {
+    color: #fbbf24;
+}
+
 /* Validation messages */
 .validation-message {
     color: #f87171;