feat(sync): device-to-device inventory sync, end to end

Wires the encrypted sync transport into a working replication between one
identity's own devices, reusing the CRDT-ready model (HLC + tombstones +
LWW) the data model was built for.

- SocialSession gains a 5th interface, sync, over the shared connection,
  built with a per-install deviceId + the app's inventory namespace.
- DeviceIdStore: a stable random per-install id in the keystore — distinct
  from the seed-derived CRDT node id, which is IDENTICAL across a user's
  devices (so it couldn't tell them apart / they'd clobber each other's
  replaceable record).
- InventorySnapshotIO (implemented by ExportImportService): build/apply the
  inventory as the SAME interchange JSON as backups but UNSEALED — the sync
  transport does the encryption, so it must not double-seal. Apply = the
  existing idempotent LWW importInventory.
- SyncService: on (re)connect publishes this device's snapshot + subscribes
  to the others'; a local edit (debounced, off tableUpdates) republishes;
  incoming snapshots merge idempotently; skips its own echo; a byte check
  skips re-publishing an unchanged snapshot so the ping-pong converges.
  Foreground only, like the inbox.
- DI/Bootstrap/switchSocialAccount wire and lifecycle it per identity.

Tests: DeviceIdStore, ExportImportService snapshot round-trip (unsealed +
idempotent), SyncService.handleRemote (applies others', skips own).

Follow-ups: deltas vs full-snapshot pushes; background sync; conflict UI.

apps/app_seeds/lib/bootstrap.dart

@@ -22,6 +22,7 @@ import 'services/social_account_store.dart';
 import 'services/social_connection.dart';
 import 'services/social_service.dart';
 import 'services/social_settings.dart';
+import 'services/sync_service.dart';
 import 'services/trust_referents.dart';
 import 'services/wot_settings.dart';
 import 'ui/theme.dart';
@@ -65,9 +66,12 @@ class _BootstrapState extends State<Bootstrap> {
     // Ask for notification permission and set up the OS channel (no-op on
     // unsupported platforms). Done before the inbox starts listening.
     if (notifications != null) await notifications.initialize();
-    // Subscribe the inbox listener BEFORE the shared connection starts
+    final sync =
+        getIt.isRegistered<SyncService>() ? getIt<SyncService>() : null;
+    // Subscribe the inbox + sync listeners BEFORE the shared connection starts
     // connecting, so the first session is caught; then bring the connection up.
     inbox?.start();
+    sync?.start();
     connection?.start();
 
     return TaneApp(