feat(developers): add timezone dropdown and rework skills section

Replace free-text timezone input with a dropdown populated from system
timezones. Replace "Why SilverLabs?" motivation section with a
skills-focused "What You Bring" section that collects what candidates
can contribute to the team, with role-specific placeholders.

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

BlazorApp/Models/DeveloperApplication.cs

@@ -36,8 +36,8 @@ public class DeveloperApplication
     [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")]
+    [Required(ErrorMessage = "Please tell us how you'll contribute")]
+    [StringLength(2000, MinimumLength = 20, ErrorMessage = "Please write at least 20 characters")]
     public string Motivation { get; set; } = string.Empty;
 }