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/app.js

@@ -100,9 +100,20 @@ modelRegister('nginx', {
 modelRegister('acme', {
     subsystem: 'acme',
     fetch: async () => {
-        const r = await apiFetch('/api/certs/list');
-        if (!r.ok) throw new Error(r.error);
-        return { certs: r.data || [] };
+        const [listR, acctR] = await Promise.allSettled([
+            apiFetch('/api/certs/list'),
+            apiFetch('/api/certs/account'),
+        ]);
+        const result = {};
+        if (listR.status === 'fulfilled' && listR.value.ok) {
+            result.certs = listR.value.data || [];
+        } else if (listR.status === 'rejected' || !listR.value.ok) {
+            throw new Error(listR.status === 'rejected' ? (listR.reason?.message || 'Failed') : (listR.value.error || 'Failed'));
+        }
+        if (acctR.status === 'fulfilled' && acctR.value.ok) {
+            result.account = acctR.value.data || { registered: false, email: '', ca: '' };
+        }
+        return result;
     },
 });