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)

backend/main.py

@@ -1933,31 +1933,17 @@ async def api_file_move(
 # Backup & Diff endpoints
 # ---------------------------------------------------------------------------
 
-def _get_backup_root() -> Path:
-    """Return the backup root directory as configured.
-    
-    If the configured path is relative, it is resolved relative to the
-    first vault's parent directory for consistency with _backup_file.
-    """
-    root = Path(os.environ.get("OBSIGATE_BACKUP_DIR", ".obsigate-backup"))
-    if not root.is_absolute():
-        # Resolve relative to vault root (inside vault, always writable)
-        try:
-            from backend.indexer import get_vault_data as _get_vd
-            vaults = index
-            if vaults:
-                first_name = next(iter(vaults.keys()))
-                vd = _get_vd(first_name)
-                if vd:
-                    root = Path(vd["path"]) / root
-        except Exception:
-            pass  # fall back to relative
-    return root
-
-
 def _get_backup_dir(vault_name: str, relative_path: str) -> Path:
-    """Return the directory where backups for a specific file are stored."""
-    backup_root = _get_backup_root()
+    """Return the directory where backups for a specific file are stored.
+    
+    Resolves relative backup paths against the SPECIFIC vault's directory,
+    matching _backup_file's resolution logic exactly.
+    """
+    backup_root = Path(os.environ.get("OBSIGATE_BACKUP_DIR", ".obsigate-backup"))
+    if not backup_root.is_absolute():
+        vault_data = get_vault_data(vault_name)
+        if vault_data:
+            backup_root = Path(vault_data["path"]) / backup_root
     return backup_root / vault_name / Path(relative_path).parent