fix(sync): harden detached launch, bounded --check probes, and pipe-clos

- `sync --ui` now launches detached; closing standalone Sync window exits app
- `--check` uses BatchMode + ConnectTimeout + 15s timeoutMs on all SSH calls
- `runSyncLauncher` settles on `exit` (not just `close`) for Electron pipe inheritance
- Added `timeoutMs` to `runSyncLauncher`; check-host passes 30s deadline
- `handleSyncCliAtEntry` wraps exit deps for testability; fixes Linux AppImage GUI fallthrough
- `config-settings-window` gains optional `onClosed` hook
- Added preload-syncui bundle to `build:syncui` script

src/main/sync-cli-entry.test.ts

@@ -0,0 +1,34 @@
+import test from 'node:test';
+import assert from 'node:assert/strict';
+import { handleSyncCliAtEntry } from './sync-cli';
+
+test('handleSyncCliAtEntry exits through the process exit dependency', async () => {
+  const exits: number[] = [];
+  const handled = await handleSyncCliAtEntry(
+    ['SubMiner', '--sync-cli', 'sync', '--make-temp'],
+    {},
+    '0.18.0',
+    {
+      run: async () => 7,
+      exit: (code) => {
+        exits.push(code);
+      },
+    },
+  );
+
+  assert.equal(handled, true);
+  assert.deepEqual(exits, [7]);
+});
+
+test('handleSyncCliAtEntry ignores normal GUI startup', async () => {
+  const handled = await handleSyncCliAtEntry(['SubMiner', '--start'], {}, '0.18.0', {
+    run: async () => {
+      throw new Error('should not run');
+    },
+    exit: () => {
+      throw new Error('should not exit');
+    },
+  });
+
+  assert.equal(handled, false);
+});