Implement Tab key host switching functionality

- Add KeyCode::Tab support to main dashboard event loop
- Add Tab key handling to TuiApp handle_input method
- Tab key now cycles to next host using existing navigate_host logic
- Host switching infrastructure was already implemented, just needed Tab key support
- Current host displayed in bold in title bar, other hosts shown normally
- Metrics filtered by selected host, full navigation working

dashboard/src/app.rs

@@ -186,6 +186,14 @@ impl Dashboard {
                                             }
                                         }
                                     }
+                                    KeyCode::Tab => {
+                                        debug!("Tab pressed - next host");
+                                        if let Some(ref mut tui_app) = self.tui_app {
+                                            if let Err(e) = tui_app.handle_input(Event::Key(key)) {
+                                                error!("Error handling tab navigation: {}", e);
+                                            }
+                                        }
+                                    }
                                     _ => {}
                                 }
                             }

dashboard/src/ui/mod.rs

@@ -112,6 +112,9 @@ impl TuiApp {
                     info!("Manual refresh requested");
                     // Refresh will be handled by main loop
                 }
+                KeyCode::Tab => {
+                    self.navigate_host(1); // Tab cycles to next host
+                }
                 _ => {}
             }
         }