feat(core): add heartbeat handling

espanso/src/cli/worker/engine/funnel/ui.rs

@@ -58,6 +58,7 @@ impl<'a> funnel::Source<'a> for UISource<'a> {
         UIEvent::ContextMenuClick(context_item_id) => {
           EventType::ContextMenuClicked(ContextMenuClickedEvent { context_item_id })
         }
+        UIEvent::Heartbeat => EventType::Heartbeat,
       },
     }
   }

espanso/src/cli/worker/mod.rs

@@ -157,6 +157,7 @@ fn worker_main(args: CliModuleArgs) -> i32 {
     .run(Box::new(move |event| {
       if let Err(error) = engine_ui_event_sender.send(event) {
         error!("unable to send UIEvent to engine: {}", error);
+        panic!("broken UI->Engine channel");
       }
     }))
     .expect("unable to run main eventloop");

espanso/src/engine/event/mod.rs

@@ -50,6 +50,7 @@ pub enum EventType {
   ProcessingError(String),
   ExitRequested(ExitMode),
   Exit(ExitMode),
+  Heartbeat,
   
   // Inputs
   Keyboard(input::KeyboardEvent),