feat(sync): Delink and Disband with master state retention

- Delink: any peer can leave; removeWriter notifies others, local state reset to defaults.
- Disband: master only; push syncGroupDisbanded so peers reset; master exports state to
  state.json, then reloads (cleanup + restorePersistedState + restorePersistedTunnels).
- Skip handleDisbandFromMaster when receiver is master so master state is not reset.
- Store syncMasterDeviceId in synced state so MASTER badge shows on all devices.
- UI: show Delink for peers, Disband for master only; confirm modals for both.

native-host/managers/sync-manager.js

@@ -213,6 +213,8 @@ async function applyRemoteUpdate() {
     if (!str) return;
     const snapshot = JSON.parse(str);
     if (snapshot.syncGroupDisbanded === true) {
+      const identity = loadIdentity();
+      if (identity && identity.isMaster) return;
       await handleDisbandFromMaster();
       return;
     }
@@ -693,8 +695,8 @@ async function delink() {
 }
 
 /**
- * Disband the sync group (master only). Pushes state with syncGroupDisbanded: true so peers
+ * Disband the sync group (master only). Exports current state to state.json, pushes
- * reset to defaults and remove their sync data. Master keeps local state but removes own sync data.
+ * syncGroupDisbanded so peers reset, then removes sync data and reloads state from state.json.
  */
 async function disband() {
   if (!storageDir || !holesailManager) return { ok: false, error: 'Storage path or holesail manager not set' };
@@ -703,8 +705,23 @@ async function disband() {
   await ensureInitialized();
   if (!pass) return { ok: false, error: 'Not linked' };
   const setStateSaveSuppressed = holesailManager.setStateSaveSuppressed;
+  const setTunnelsRestoredPromiseFn = setTunnelsRestoredPromise;
+  const restorePersistedTunnelsFn = restorePersistedTunnels;
   try {
+    const statePath = getStateFilePath();
     const snapshot = holesailManager.getStateSnapshot();
+    const deviceNames = readDeviceNamesFromState();
+    if (Object.keys(deviceNames).length > 0) snapshot.deviceNames = deviceNames;
+    delete snapshot.syncGroupDisbanded;
+    if (statePath) {
+      try {
+        fs.writeFileSync(statePath, JSON.stringify(snapshot, null, 2), 'utf8');
+        log('Sync: state exported to state.json');
+      } catch (e) {
+        if (process.stderr) process.stderr.write('[sync-manager] disband export failed: ' + e.message + '\n');
+        return { ok: false, error: e.message };
+      }
+    }
     snapshot.syncGroupDisbanded = true;
     if (typeof setStateSaveSuppressed === 'function') setStateSaveSuppressed(true);
     try {
@@ -716,8 +733,18 @@ async function disband() {
     await closePass();
     removeAutopassDir();
     clearIdentity();
+    initPromise = null;
     lastSyncedAt = null;
-    log('Sync: group disbanded by master');
+    if (setTunnelsRestoredPromiseFn && restorePersistedTunnelsFn) {
+      await holesailManager.cleanup().catch(() => {});
+      const restored = holesailManager.restorePersistedState();
+      const restorePromise = restorePersistedTunnelsFn(holesailManager, restored).catch((e) =>
+        log('Sync: post-disband restore failed:', e.message)
+      );
+      setTunnelsRestoredPromiseFn(restorePromise);
+      await restorePromise;
+    }
+    log('Sync: group disbanded by master; state reloaded from state.json');
     return { ok: true };
   } catch (e) {
     if (typeof setStateSaveSuppressed === 'function') setStateSaveSuppressed(false);