feat(fuse,core,cli,gui): move another computer's backup to this one

A user switching from a Windows PC to Linux had two half answers:
adopting the old device makes this machine back up as it, and restoring
its folders keeps them in the old device's tree. Neither puts the data
under this machine's own device.

Device roots are separate shares on the main volume, so each top-level
folder of the other device can move under this device's root in one
server-side move. Nothing is uploaded again, and the other device stays
registered, without those folders.

- core: `Request::MigrateDevice { uid }` and
  `Db::sync_folder_set_share`
- fuse: `Core::migrate_device` moves every folder, suffixes a name
  already taken here with the other device's name, keeps going past a
  failed folder, and re-points sync folders that already tracked a
  moved folder at the new share
- cli: `pdfs devices migrate UID [--yes]` confirms, moves, then runs
  the ordinary restore picker
- gui: "Move to This Computer…" on another computer's row, followed by
  the restore picker
- i18n: translate the new strings in every catalog
- docs: CLI.md, USER_GUIDE.md, RECOVERY.md, ARCHITECTURE.md, changelog

crates/pdfs-gui/src/app/pages/devices.rs

@@ -439,6 +439,10 @@ fn other_computer_row(ui: &Rc<Ui>, dev: &DeviceInfo) -> adw::ActionRow {
     menu.item(&gettext("Restore to This Computer…"), move || {
         prompt_restore_folders(&ui_c, Some((uid_c.clone(), name_c.clone())))
     });
+    let (ui_c, uid_c, name_c) = (ui.clone(), uid.clone(), name.clone());
+    menu.item(&gettext("Move to This Computer…"), move || {
+        prompt_migrate_device(&ui_c, &uid_c, &name_c)
+    });
     menu.section();
     let (ui_c, uid_c, name_c) = (ui.clone(), uid.clone(), name.clone());
     menu.item(&gettext("Rename…"), move || {
@@ -1096,6 +1100,67 @@ pub(crate) fn prompt_adopt_device(ui: &Rc<Ui>, uid: &str, name: &str) {
     dialog.present(win.as_ref());
 }
 
+/// Ask before moving another computer's folders into this computer's backup,
+/// then offer them for syncing here.
+///
+/// Destructive-looking on purpose: the other computer keeps its device but
+/// loses its folders, and a copy of Proton Drive still running there would
+/// watch them vanish.
+pub(crate) fn prompt_migrate_device(ui: &Rc<Ui>, uid: &str, name: &str) {
+    let win = ui_window(ui);
+    let dialog = adw::AlertDialog::builder()
+        .heading(gettext("Move Backup to This Computer?"))
+        // Translators: {name} is the name of the other computer whose folders move to this one.
+        .body(gettext_f(
+            "The folders backed up by “{name}” move into this computer's backup in Proton Drive, and you choose where they go on this computer. “{name}” stays in your account, without those folders.\n\nStop Proton Drive on “{name}” first. Otherwise it sees its folders disappear.",
+            &[("name", name)],
+        ))
+        .build();
+    dialog.add_response("cancel", &gettext("Cancel"));
+    dialog.add_response("move", &gettext("Move Here"));
+    dialog.set_response_appearance("move", adw::ResponseAppearance::Destructive);
+    dialog.set_default_response(Some("cancel"));
+    dialog.set_close_response("cancel");
+    let ui = ui.clone();
+    let uid = uid.to_string();
+    // Translators: {name} is the name of the computer whose folders moved to this one.
+    let done = gettext_f("Moved the backup of “{name}”", &[("name", name)]);
+    dialog.connect_response(None, move |_, resp| {
+        if resp != "move" {
+            return;
+        }
+        ui.busy_begin();
+        let rx = spawn_request(
+            ui.dirs.control_socket(),
+            Request::MigrateDevice { uid: uid.clone() },
+        );
+        let (ui, done) = (ui.clone(), done.clone());
+        glib::spawn_future_local(async move {
+            let result = rx.recv().await;
+            ui.busy_end();
+            let failed = gettext("Couldn't move that backup");
+            match result {
+                Ok(Ok(Response::Ok { .. })) => {
+                    reload_sync_pages(&ui);
+                    toast(&ui, &done);
+                    // The folders are this computer's now, so its own restore
+                    // picker is where the user chooses their local paths.
+                    prompt_restore_folders(&ui, None);
+                }
+                Ok(Ok(Response::Error { message, kind })) => {
+                    toast_failure(&ui, &failed, &message, kind)
+                }
+                _ => toast_error(
+                    &ui,
+                    &failed,
+                    &gettext("The Proton Drive service didn't respond."),
+                ),
+            }
+        });
+    });
+    dialog.present(win.as_ref());
+}
+
 /// Run a mutation raised from Computers or Locations and reload the page on success.
 pub(crate) fn run_devices_mutation(ui: &Rc<Ui>, req: Request, done: &str, failed: &str) {
     ui.busy_begin();