fix(watcher): forward notify backend errors instead of silently discarding

Previously Err(_) => return in the notify callback silently dropped all
backend errors (inotify exhaustion, fs unmount, permission loss), leaving
the application unaware that file monitoring had stopped working.

Add FileEvent::WatcherError { message: String } variant to propagate
backend errors through the existing bounded channel. The TUI consumer
receives the event without disrupting the UI for transient errors.

Closes #14

crates/tui/src/app.rs

@@ -857,6 +857,7 @@ impl App {
                 FileEvent::Removed => {
                     self.loading_state = AppLoadingState::Error("File has been deleted".into());
                 }
+                FileEvent::WatcherError { message: _ } => {}
             }
         }
     }