Replace flat paginated document list with collapsible tree view.
Backend: two-query-per-level approach (files + DISTINCT deeper paths
for virtual folder derivation), path parsing in Go for PG+SQLite
compatibility, text_pattern_ops index for prefix range scans.
Frontend: VaultTree component with doc_type colored icons, compact
single-line nodes, truncate-middle filenames, scope dots, hover
tooltip for date. Sidebar expanded to 384px on large screens.
Also fixes pre-existing SQLite scanVaultDocRow missing path_basename.
