feat(developers): simplify timezone dropdown and make email optional

Replace 100+ raw system timezones with curated list of 26 major zones
with browser auto-detection via Intl API. Remove email requirement since
applicants receive a @silverlabs.uk address — fallback to username@silverlabs.uk
when no personal email is provided.

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

BlazorApp/Models/DeveloperApplication.cs

@@ -8,9 +8,8 @@ public class DeveloperApplication
     [StringLength(100, MinimumLength = 2)]
     public string FullName { get; set; } = string.Empty;
 
-    [Required(ErrorMessage = "Email is required")]
     [EmailAddress(ErrorMessage = "Invalid email address")]
-    public string Email { get; set; } = string.Empty;
+    public string? Email { get; set; }
 
     [Required(ErrorMessage = "Username is required")]
     [RegularExpression(@"^[a-zA-Z0-9_-]{3,30}$", ErrorMessage = "Username must be 3-30 characters, letters, numbers, hyphens and underscores only")]