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/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.