Show refreshing status during library scan

- Add is_refreshing flag to AppState
- Display 'Refreshing library...' in status panel during scan
- Note: scan is currently blocking, message may be brief
- Infrastructure ready for future async scanning

src/state/mod.rs

@@ -26,6 +26,7 @@ pub struct AppState {
     pub marked_files: HashSet<PathBuf>,
     pub playlist: Vec<PathBuf>,
     pub playlist_index: usize,
+    pub is_refreshing: bool,
 }
 
 #[derive(Debug, Clone)]
@@ -58,6 +59,7 @@ impl AppState {
             marked_files: HashSet::new(),
             playlist: Vec::new(),
             playlist_index: 0,
+            is_refreshing: false,
         }
     }