feat(developers): overhaul signup to auto-register SilverDESK accounts

Users now pick a password and get a SilverDESK account immediately on
submit. The form includes debounced username availability checking,
password fields with validation, and a post-submit link to SilverDESK.
The approval flow no longer creates a SilverDESK user (already exists)
and only provisions Mattermost + Mailcow.

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

BlazorApp/Services/ProvisioningService.cs

@@ -20,17 +20,15 @@ public class ProvisioningService
         var results = new List<string>();
         var allSuccess = true;
 
-        // 1. Create SilverDESK user
-        var (deskOk, deskMsg) = await CreateSilverDeskUserAsync(username, email, fullName);
-        results.Add($"SilverDESK: {deskMsg}");
-        if (!deskOk) allSuccess = false;
+        // SilverDESK user already exists (created at application time)
+        // Only provision external services
 
-        // 2. Create Mattermost user
+        // 1. Create Mattermost user
         var (mmOk, mmMsg) = await CreateMattermostUserAsync(username, email, fullName);
         results.Add($"Mattermost: {mmMsg}");
         if (!mmOk) allSuccess = false;
 
-        // 3. Create Mailcow mailbox
+        // 2. Create Mailcow mailbox
         var (mailOk, mailMsg) = await CreateMailcowMailboxAsync(username, fullName);
         results.Add($"Mailcow: {mailMsg}");
         if (!mailOk) allSuccess = false;