fix: rewrite backup viewer close button — match editor-btn pattern

- Close button: <button class="backup-close-btn"><i data-lucide="x"></i></button>
  (same HTML pattern as AdminPanel editor-btn)
- Handler: addEventListener via DOM query in setBody() after innerHTML
  (same JS pattern as AdminPanel)
- Removed all inline onclick, global capture handler, window._backupViewerClose
- CSS: .backup-close-btn now matches .editor-btn style (bordered icon button)
- Escape key + overlay click still work for closing

frontend/style.css

@@ -7250,24 +7250,24 @@ body.popup-mode .content-area {
 }
 
 .backup-close-btn {
-  width: 32px;
-  height: 32px;
-  border: none;
+  width: 36px;
+  height: 36px;
+  padding: 0;
+  border: 1px solid var(--border);
   border-radius: 6px;
   background: transparent;
-  color: var(--text-muted);
-  font-size: 1.4rem;
+  color: var(--text-secondary);
   cursor: pointer;
-  display: flex;
+  transition: all 150ms ease;
+  display: inline-flex;
   align-items: center;
   justify-content: center;
-  line-height: 1;
-  transition: color 0.15s, background 0.15s;
+  flex: 0 0 auto;
 }
 
 .backup-close-btn:hover {
-  color: var(--text-primary);
-  background: var(--bg-tertiary);
+  color: var(--accent);
+  border-color: var(--accent);
 }
 
 .backup-toolbar {