feat(fuse,core,cli,gui): make folders available offline from the app

The daemon has kept recursive folder pins for a long time, but the app
offered "Available offline" for files only. The context menu, the
details pane and the selection bar now offer it for folders too, and a
pinned folder carries the offline badge in the browser and the launcher.

A folder pin used to download the whole subtree inside the control
request, so a large folder outlasted the client's 120 s read timeout,
and every pinned file was read into memory before it was cached. The
daemon now records the pin, answers at once and downloads on a
background thread as a job the tray and the app show, stopping when the
folder is unpinned or the daemon stops. Pins of files and folders both
go through fetch_content, which streams to disk and skips fresh blobs.
A folder pin is refused while offline instead of looking kept.

A file kept only because a folder above it is pinned has no pin of its
own, and unpinning it silently did nothing. ListDir now reports
pinned_by_folder; the app shows such an entry as available offline with
the toggle greyed out, and the daemon refuses the unpin with a message
naming the folder. Unpinning a folder no longer evicts files that are
pinned on their own.

Requested on Reddit.

docs/CHANGELOG.md

@@ -16,6 +16,12 @@ scratch (user data in `staging/` and `recovery/` is never touched by this).
   `proton-drive-linux.cachix.org`, so NixOS installs download the client instead of compiling
   it. See [Install → Binary cache](INSTALL.md#binary-cache).
   ([#37](https://github.com/narrrl/proton-drive-linux/issues/37))
+- **Folders can be made available offline in the app.** The context menu, the details pane and
+  the selection bar now offer it for folders too. Every file in the folder and its subfolders
+  downloads in the background with its progress in the tray and the app, and pinned folders carry
+  the offline badge. A file kept through its folder shows as available offline but cannot be
+  made online only on its own. `pdfs pin` on a folder now returns at once instead of timing out
+  on a large folder, and pinned files download to disk instead of into memory.
 
 ### Fixed
 - **Removing the package left the drive mounted, and deletes there still reached Proton Drive.**