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/Models/DeveloperApplication.cs

@@ -28,6 +28,14 @@ public class DeveloperApplication
 
     public string? Skills { get; set; }
 
+    [Required(ErrorMessage = "Password is required")]
+    [StringLength(100, MinimumLength = 8, ErrorMessage = "Password must be at least 8 characters")]
+    public string Password { get; set; } = string.Empty;
+
+    [Required(ErrorMessage = "Please confirm your password")]
+    [Compare("Password", ErrorMessage = "Passwords do not match")]
+    public string ConfirmPassword { get; set; } = string.Empty;
+
     [Required(ErrorMessage = "Please tell us why you want to join")]
     [StringLength(2000, MinimumLength = 20, ErrorMessage = "Motivation must be between 20 and 2000 characters")]
     public string Motivation { get; set; } = string.Empty;