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/stats-sync/sync-flow.ts

@@ -145,11 +145,12 @@ export function withJsonEvents(deps: SyncFlowDeps): SyncFlowDeps {
   };
 }
 
-export function runSnapshotMode(
+export async function runSnapshotMode(
   context: SyncFlowContext,
   dbPath: string,
   deps: SyncFlowDeps,
-): void {
+): Promise<void> {
+  await deps.ensureTrackerQuiescent(context, dbPath);
   const outPath = deps.resolvePath(context.args.syncSnapshotPath);
   deps.emitEvent({
     type: 'stage',
@@ -409,7 +410,10 @@ export async function runHostSync(
   }
 }
 
-export async function runSyncFlow(context: SyncFlowContext, inputDeps: SyncFlowDeps): Promise<boolean> {
+export async function runSyncFlow(
+  context: SyncFlowContext,
+  inputDeps: SyncFlowDeps,
+): Promise<boolean> {
   let deps = inputDeps;
   const { args } = context;
   if (!args.sync) return false;
@@ -430,7 +434,7 @@ export async function runSyncFlow(context: SyncFlowContext, inputDeps: SyncFlowD
     if (args.syncCheck) {
       await runCheckMode(context, deps);
     } else if (args.syncSnapshotPath) {
-      runSnapshotMode(context, dbPath, deps);
+      await runSnapshotMode(context, dbPath, deps);
     } else if (args.syncMergePath) {
       await runMergeMode(context, dbPath, deps);
     } else if (args.syncHost) {