feat(ui): browse and recover by folder

A Folders mode next to grid and list shows one level at a time: subfolders
first with their file count and size, then the files, under a clickable
breadcrumb. Folders and files are marked with tri-state boxes that persist
while navigating; the bar shows the exact total from the core, and the
selection is recovered relative to each marked folder. The sidebar folder
tree navigates in this mode.

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

crates/guara-core/tests/disk.rs

@@ -783,6 +783,7 @@ fn navegacao_por_pastas_e_recuperacao_da_selecao() {
         folders: vec!["Documentos/Trabalho".into()],
         ids: vec![id_of("_raia.jpg")],
         exclude: vec![id_of("tese.docx")],
+        exclude_folders: vec![],
     };
     let out = tempfile::tempdir().unwrap();
     let rj = eng