feat(qconnect): own remote-sync Mutex on QconnectApp via new(transport, sink, sync) + sync_handle (slice 2+4)

QconnectApp now holds the shared QconnectRemoteSyncState accumulator behind
its own Mutex, disjoint from `state` (the two are never co-locked). The Tauri
adapter still builds the Arc and passes the SAME clone to its event sink /
service loop and into new(), so there is exactly one lock backing the
session/liveness/renderer paths -- the app simply gains a handle to it via
sync_handle(). Pure wiring, byte-identical locking, no logic moved. Prepares
slices 2+4.4/5 to relocate the session-apply + watchdog logic into qconnect-app.

cc

src-tauri/src/qconnect/service.rs

@@ -304,7 +304,11 @@ impl QconnectServiceState {
             sync_state: Arc::clone(&sync_state),
             app: Arc::new(std::sync::OnceLock::new()),
         });
-        let app = Arc::new(QconnectApp::new(transport, Arc::clone(&sink)));
+        let app = Arc::new(QconnectApp::new(
+            transport,
+            Arc::clone(&sink),
+            Arc::clone(&sync_state),
+        ));
         // P1-6: wire the owning app into the sink so it can emit reports
         // (e.g. is_active=true after SetActive(true)).
         sink.set_app(&app);