feat: add ACME account management with validation pipeline

- Register, view, and deactivate ACME accounts via API and UI
- 16-check validation framework for certificate issuance readiness
- DNS resolution, port, nginx, and firewall pre-flight checks
- External IP detection with NAT support and fallback providers
- Account card and settings modal in certificates page
- Guard certificate issuance behind account registration
- Update modal CSS to overlay-based approach
- 1000+ lines of tests for validation and account handlers

webui/static/style.css

@@ -367,7 +367,7 @@ body {
 }
 
 /* Modal */
-.modal {
+.modal-overlay {
     position: fixed;
     inset: 0;
     background: rgba(0, 0, 0, 0.6);
@@ -380,12 +380,16 @@ body {
     transition: opacity 0.2s, visibility 0.2s;
 }
 
-.modal.show {
+.modal-overlay.active {
     opacity: 1;
     visibility: visible;
 }
 
-.modal-content {
+.modal-overlay.active .modal {
+    transform: scale(1);
+}
+
+.modal-overlay .modal {
     background: var(--bg-secondary);
     border: 1px solid var(--border);
     border-radius: 10px;
@@ -397,10 +401,13 @@ body {
     transition: transform 0.2s;
 }
 
-.modal.show .modal-content {
-    transform: scale(1);
+.modal-title {
+    margin: 0 0 1rem;
+    font-size: 1.1rem;
 }
 
+
+
 /* Toggle Switch */
 .toggle-switch {
     position: relative;