fix(hotkey): exit listener cleanly when event channel is dropped

The evdev listener's run loop did `let _ = event_tx.send(event).await`
inside the trigger-key match arm. If the receiver was dropped without
the explicit shutdown signal (set hotkey to None), the send returned
Err and the loop kept polling — sending into a closed channel forever
until something else terminated the task.

Replace with explicit handling: on Err, log via log::warn! once and
return Ok(()) from `run`. The shutdown-via-None path is unaffected.
kon-hotkey still 4/4.

https://claude.ai/code/session_0189xUb6ie6t9qHkzatGZ9Rb

crates/hotkey/src/linux.rs

@@ -317,10 +317,26 @@ async fn device_listener(
                                     && alt_held == combo.alt
                                     && super_held == combo.super_key
                                 {
-                                    if pressed {
-                                        let _ = event_tx.send(HotkeyEvent::Pressed).await;
+                                    let to_send = if pressed {
+                                        Some(HotkeyEvent::Pressed)
                                     } else if released {
-                                        let _ = event_tx.send(HotkeyEvent::Released).await;
+                                        Some(HotkeyEvent::Released)
+                                    } else {
+                                        None
+                                    };
+                                    if let Some(event) = to_send {
+                                        if event_tx.send(event).await.is_err() {
+                                            // Receiver was dropped without an
+                                            // explicit None-on-hotkey-rx
+                                            // shutdown. Log once and exit so
+                                            // the listener doesn't spin
+                                            // sending into a closed channel.
+                                            log::warn!(
+                                                "Hotkey event channel closed; \
+                                                 listener for device exiting"
+                                            );
+                                            return Ok(());
+                                        }
                                     }
                                 }
                             }