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.

docs/CHANGELOG.md

@@ -78,6 +78,11 @@ Schema: **31** (unchanged).
   instead of covering them. It has Info, Offline, Sharing and Versions sections: who has access or
   whether there is a public link, how many versions there are, and Share…, Copy Link and
   Versions… buttons.
+- **Search in this folder.** Inside a folder, a folder button beside the search box limits the search
+  to that folder and everything below it. `pdfs search --in <folder>` does the same from the CLI.
+  In the list view a Location column shows which folder each hit is in. Pinned files listed by the
+  search prompt and `pdfs-query` show whether they are on disk, instead of always showing them as
+  not downloaded.
 - **US spelling throughout.** The app, the CLI and the docs say "Favorite" and "color", matching the
   `pdfs favorite` command.