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

docs/ARCHITECTURE.md

@@ -377,6 +377,12 @@ Restore lists the device's folders, proposes a local path from the profile, and
 adds a synced folder bound to the remote uid. The download is an ordinary pass against an empty
 baseline. The user side is in [RECOVERY.md](RECOVERY.md).
 
+`Core::migrate_device` moves another device's top-level folders under this machine's device root.
+Device roots are separate shares on the main volume, so each folder is one server-side move that
+keeps its uid. A name already taken here gets the other device's name as a suffix. A synced folder
+that already tracked one of them keeps its `remote_uid` and only has its `remote_share_id`
+re-pointed. The moved folders are then offered by the ordinary restore.
+
 ---
 
 ## 8. Control protocol

docs/CHANGELOG.md

@@ -11,6 +11,13 @@ scratch (user data in `staging/` and `recovery/` is never touched by this).
 
 ## [Unreleased]
 
+### Added
+- **Move another computer's backup to this one.** **Move to This Computer…** on the Computers page,
+  or `pdfs devices migrate UID`, moves every folder another computer backs up, for example an old
+  Windows PC, into this computer's backup on Proton Drive. Then it offers them for syncing here. The
+  folders move on Proton Drive itself, so nothing is uploaded again. The other computer stays in the
+  account, without those folders.
+
 ## [3.1.2] — 2026-10-08
 
 Schema: **42**, unchanged.

docs/CLI.md

@@ -103,6 +103,7 @@ commands are for scripts and for cases where the mount is not available.
 | `pdfs devices rename UID NAME` | Rename a computer. |
 | `pdfs devices rm UID` | Remove a computer and its backup from the account. |
 | `pdfs devices adopt UID` | Make this machine continue another computer's backup, for example after a reinstall. `--clear` returns to matching by hostname. |
+| `pdfs devices migrate UID [--yes]` | Move another computer's folders into this computer's backup, then restore them here like `pdfs sync restore`. The other computer stays registered, without those folders. `--yes` skips the confirmation and accepts every proposed path. See [RECOVERY.md](RECOVERY.md). |
 
 ## Trash
 

docs/RECOVERY.md

@@ -133,6 +133,29 @@ pdfs sync restore --device UID
 The folders stay part of the other computer's backup and sync in both directions. This machine
 does not take over that computer's backup.
 
+### Moving another computer's backup here
+
+To switch computers for good, for example from a Windows PC to this Linux machine, move the old
+computer's folders into this computer's backup instead. Use **Move to This Computer…** on that
+computer's row, or:
+
+```bash
+pdfs devices list                 # find the old computer's UID
+pdfs devices migrate UID
+```
+
+Each folder moves on Proton Drive itself, so nothing is uploaded again and no storage is used
+twice. A folder whose name this computer already uses gets the old computer's name added, for
+example `Documents (DESKTOP-1)`. The restore picker then opens, as in
+[Step 3](#step-3-restore-the-folders), so you choose where each folder goes here.
+
+The old computer stays in your account, without those folders. Unlike
+[adoption](#step-2-continue-the-old-computers-backup), this machine keeps its own name and backup.
+
+> [!WARNING]
+> Stop Proton Drive on the old computer first. A client still running there sees its folders
+> disappear, and may upload them again as new.
+
 ## 3. How the restore works
 
 This section is for the curious and for anyone debugging a restore.

docs/USER_GUIDE.md

@@ -278,6 +278,9 @@ Proton Drive keeps earlier versions of every file. **Details → Versions…** i
   **Restore to This Computer…** downloads its folders here.
 - **Continue This Backup Here…** makes this computer take over another computer's backup, for
   example after reinstalling or replacing a machine. See [RECOVERY.md](RECOVERY.md).
+- **Move to This Computer…** moves another computer's folders into this computer's backup, for
+  example when you switch from Windows to Linux, and then offers them for syncing here. The other
+  computer stays in the list, without those folders.
 
 ## Search launcher