feat(sync-ui): defer app quit until async cleanup resolves

- check-host participates in active-run coordination and can be cancelled
- shutdown() cancels and awaits the active launcher run on quit
- onWillQuit calls preventDefault and re-triggers quit once cleanup settles
- Snapshot mode awaits tracker quiescent before writing output
- Auto-scheduler catches synchronous triggerHostSync failures
- SCP endpoint accepts Windows absolute paths; cmd.exe rejects % in quoted values
- runAppCommand unified (inherit vs pipe stdio) in launcher/mpv.ts
- Docs: add --check, --json, --ui, --sync-cli, --make-temp/--remove-temp examples

src/core/services/app-lifecycle.test.ts

@@ -140,6 +140,38 @@ test('startAppLifecycle still acquires lock for startup commands', () => {
   assert.equal(getLockCalls(), 1);
 });
 
+test('startAppLifecycle defers quit until async cleanup settles', async () => {
+  let willQuit: ((event: { preventDefault(): void }) => void) | null = null;
+  let releaseCleanup: (() => void) | null = null;
+  const cleanupDone = new Promise<void>((resolve) => {
+    releaseCleanup = resolve;
+  });
+  let prevented = false;
+  const { deps, calls } = createDeps({
+    shouldStartApp: () => true,
+    onWillQuit: (handler) => {
+      willQuit = handler;
+    },
+    onWillQuitCleanup: () => cleanupDone,
+  });
+
+  startAppLifecycle(makeArgs({ start: true }), deps);
+  assert.ok(willQuit);
+  (willQuit as (event: { preventDefault(): void }) => void)({
+    preventDefault: () => {
+      prevented = true;
+    },
+  });
+  assert.equal(prevented, true);
+  assert.deepEqual(calls, []);
+
+  assert.ok(releaseCleanup);
+  (releaseCleanup as () => void)();
+  await cleanupDone;
+  await Promise.resolve();
+  assert.deepEqual(calls, ['quitApp']);
+});
+
 test('startAppLifecycle app ping exits non-zero immediately when no running instance owns the lock', () => {
   const { deps, calls, getLockCalls } = createDeps({
     shouldStartApp: () => false,
@@ -252,7 +284,7 @@ test('startAppLifecycle routes control socket commands through the second-instan
     },
   });
 
-  let willQuitHandler: (() => void) | null = null;
+  let willQuitHandler: ((event: { preventDefault(): void }) => void) | null = null;
   deps.onWillQuit = (handler) => {
     willQuitHandler = handler;
   };
@@ -274,7 +306,7 @@ test('startAppLifecycle routes control socket commands through the second-instan
   assert.deepEqual(handled, ['ready', 'second-instance:start']);
 
   assert.ok(willQuitHandler);
-  (willQuitHandler as () => void)();
+  (willQuitHandler as (event: { preventDefault(): void }) => void)({ preventDefault: () => {} });
   assert.deepEqual(handled, ['ready', 'second-instance:start', 'control-close']);
 });