feat(core,fuse,gui,cli): search scope and location column

Request::Search takes an optional folder scope (serde default, so older
front-ends keep searching everywhere). The GUI offers a This folder toggle
beside the search box and shows a Location column for hits; the CLI gains
pdfs search --in. Pins report whether a pinned file is cached, so pin-derived
search hits badge correctly.

crates/pdfs-gui/src/query.rs

@@ -45,7 +45,7 @@ pub(crate) fn pins(socket: &Path) -> Result<Vec<Hit>, String> {
                 size: 0,
                 modified: 0,
                 pinned: true,
-                cached: false,
+                cached: pin.cached,
                 uid: pin.uid,
                 mounted_path: None,
                 score: 0,