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/app/src/state.rs

@@ -10,6 +10,8 @@ use std::sync::Arc;
 /// Things a feature asks the main window to do (handled once per frame, in order).
 #[allow(clippy::large_enum_variant)] // a per-frame queue; boxing buys nothing
 pub enum Request {
+    /// Merge profiles with the sync folder (manual, or after a change).
+    SyncProfiles,
     /// Open the profile editor for a new profile (optionally pre-set protocol / folder).
     NewProfile {
         protocol: Option<Protocol>,