feat(site-builder): detect two-tab save conflicts; Reload latest / Keep mine modal

Refs #31

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

craft/src/styles/editor.css

@@ -2020,3 +2020,36 @@ body {
   cursor: pointer;
   padding: 0 4px;
 }
+
+.conflict-backdrop {
+  position: fixed;
+  inset: 0;
+  z-index: var(--z-modal);
+  display: flex;
+  padding: 16px;
+  background: rgba(15, 23, 42, 0.7);
+}
+/* Another-tab conflict modal */
+.conflict-modal {
+  max-width: 420px;
+  margin: auto;
+  padding: 20px;
+  background: #1e293b;
+  border: 1px solid #475569;
+  border-radius: 8px;
+  color: #e2e8f0;
+  font-size: 14px;
+}
+.conflict-modal h3 { margin: 0 0 8px; font-size: 16px; }
+.conflict-modal p { margin: 0 0 8px; }
+.conflict-modal-error { color: #fecaca; }
+.conflict-modal-actions { display: flex; gap: 8px; justify-content: flex-end; margin-top: 12px; }
+.conflict-modal-actions button {
+  background: none;
+  border: 1px solid #94a3b8;
+  border-radius: 4px;
+  color: inherit;
+  cursor: pointer;
+  padding: 4px 12px;
+}
+.conflict-modal-actions button:disabled { opacity: 0.5; cursor: default; }