fix(host-control-mode): remove redundant CONTENT_EVENT guard, persist desync in storage, block stale tab HCM_DESYNC_STATE

Original code at L1957 already validates sender.tab tabId against
currentTabId before calling processEvent() — added guard was
duplicate. HCM_DESYNC_STATE had no such guard; added with same
sender.tab && currentTabId pattern (popup-safe via undefined
short-circuit).

hcmDesynced now persisted in session storage:
- Restored in ensureState()
- Set on HCM_DESYNC_STATE change
- Reset to false in all 3 teardown storage.sets
Survives SW restarts; cleared on leave/switch/idle-room

extension/background.js

@@ -1857,13 +1857,6 @@ async function handleAsyncMessage(message, sender, sendResponse) {
             }
         });
     } else if (message.type === 'CONTENT_EVENT') {
-        // Only process media events from the currently selected tab — ignore
-        // stale content scripts in previously-selected tabs. Popup-originated
-        // events (no sender.tab) always pass through.
-        if (sender.tab && currentTabId && currentTabId !== sender.tab.id) {
-            sendResponse({ status: 'ignored_unselected_tab' });
-            return;
-        }
         const processEvent = () => {
             // Host Control Mode (sender-side): a guest in host-only mode must not
             // drive the room. Don't broadcast; hand the action back to content.js so