fix(developers): check username on blur instead of keystroke to avoid rate limiting

SilverDESK rate-limits /api/auth/check-username after ~2 requests with a
5-minute cooldown. The old 500ms debounce per keystroke quickly exhausted
this limit, breaking the form. Now checks only on field blur, validates
format client-side while typing, and caches results to skip redundant calls.

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

BlazorApp/wwwroot/developers-styles.css

@@ -209,6 +209,10 @@
     color: #fbbf24;
 }
 
+.username-format-error {
+    color: #fb923c;
+}
+
 /* Validation messages */
 .validation-message {
     color: #f87171;