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.

rust/core/src/models.rs

@@ -628,6 +628,8 @@ pub struct AppPreferences {
     pub confirm_multiline_paste: bool,
     /// Reopen the previous session's tabs and splits at launch.
     pub restore_sessions: bool,
+    /// Folder holding the encrypted profile sync file (empty: sync off).
+    pub sync_folder: String,
 }
 
 impl Default for AppPreferences {
@@ -650,6 +652,7 @@ impl Default for AppPreferences {
             allow_remote_clipboard: true,
             confirm_multiline_paste: true,
             restore_sessions: true,
+            sync_folder: String::new(),
         }
     }
 }