feat(core,fuse,gui,cli): restore to this computer per device

ListDeviceRestorableFolders and RestoreDeviceFolders read another
registered device's folders and attach them with that device's share,
without changing this machine's cached device. The Computers page moves
restore into each computer's menu, and pdfs sync restore takes --device.

docs/CHANGELOG.md

@@ -46,6 +46,11 @@ Schema: **31** (unchanged).
 - **Stop sharing.** Shared by Me → item menu → "Stop Sharing…" and `pdfs unshare <path>` (with no
   id) remove the public link, every invitation and every member in one go. Each step is tried even
   if an earlier one fails, and a partial result names what is still shared.
+- **Restore another computer's folders.** Every computer's menu on the Computers page has
+  "Restore to This Computer…", which syncs that computer's backed-up folders down to paths you
+  pick; they keep syncing with that computer's copy, and this machine keeps its own identity.
+  This computer's own restore moved from the header into its row's menu. `pdfs sync restore
+  --device <uid>` does the same from the CLI.
 - **Public link expiry.** The Share dialog's create-link form has an "Expires" choice (Never,
   1 day, 7 days, 30 days), and an existing link shows when it expires or that it has expired.