fix: backup listing uses same vault for read/write + global close handler

- _get_backup_dir: now resolves relative path using the SPECIFIC vault's
  directory (matching _backup_file exactly), not the first vault in index
- Removed _get_backup_root (unused after refactor)
- backups.js: global document click handler for #backup-close (same
  pattern as Graph View's initGraphView)

frontend/js/backups.js

@@ -14,6 +14,12 @@ import { showToast } from './ui.js';
 import { refreshSidebarTreePreservingState } from './sidebar.js';
 import { safeCreateIcons } from './utils.js';
 
+// Global handler: same pattern as Graph View's initGraphView()
+document.addEventListener('click', function(e) {
+  const btn = e.target.closest('#backup-close');
+  if (btn && state.overlay) close();
+});
+
 // ---------------------------------------------------------------------------
 // State
 // ---------------------------------------------------------------------------