feat: encrypted profile sync through a shared folder

Profiles can be kept in step across machines through any folder they
share (cloud drive, network share, git checkout): one file,
nexus-profiles.nexus, in the encrypted-export format under a sync
password kept in the keychain/vault. Each sync merges profile by profile
(newest updated_at wins), propagates deletions as dated tombstones (an
edit after a deletion brings the profile back; tombstones expire after
180 days), validates every incoming profile, writes the file only when
it changed, and never syncs credentials.

Settings → Security → Profile sync picks the folder and sets the
password; syncs run at launch, 3 s after a profile change, every 10
minutes and on Sync Now (automatic runs stay quiet unless something
changed). An end-to-end test drives create, edit and delete between two
data directories sharing a folder.

docs/overview.md

@@ -41,6 +41,7 @@ Use Nexus Terminal when you need to:
 - Multi-select profiles with Ctrl/Cmd-click for bulk connect, disconnect, tagging, moving between groups and deleting.
 - Test basic host and port reachability before connecting.
 - Import existing OpenSSH config entries from `~/.ssh/config`, including hostnames, usernames, ports, identity files and jump hosts.
+- Sync profiles between machines through any shared folder, as one encrypted file (newest change wins; deletions propagate; credentials stay local).
 - Export and import profiles using encrypted `.nexus` bundles.
 - Plain JSON export and import, with secrets redacted unless you choose to include them.
 - Choose import behaviour for duplicates: skip, overwrite or duplicate.