Add refresh shortkey 'r' for on-demand metrics refresh

Implements ZMQ command protocol for dashboard-to-agent communication:
- Agents listen on port 6131 for REQ/REP commands
- Dashboard sends "refresh" command when 'r' key is pressed
- Agents force immediate collection of all metrics via force_refresh_all()
- Fresh data is broadcast immediately to dashboard
- Updated help text to show "r: Refresh all metrics"

Also includes metric-level caching architecture foundation for future
granular control over individual metric update frequencies.

dashboard/src/app.rs

@@ -69,6 +69,7 @@ pub struct App {
     active_host_index: usize,
     show_help: bool,
     should_quit: bool,
+    refresh_requested: bool,
     last_tick: Instant,
     tick_count: u64,
     status: String,
@@ -106,6 +107,7 @@ impl App {
             active_host_index: 0,
             show_help: false,
             should_quit: false,
+            refresh_requested: false,
             last_tick: Instant::now(),
             tick_count: 0,
             status,
@@ -138,7 +140,8 @@ impl App {
                 self.status = "Exiting…".to_string();
             }
             KeyCode::Char('r') | KeyCode::Char('R') => {
-                self.status = "Manual refresh requested".to_string();
+                self.refresh_requested = true;
+                self.status = "Refresh requested - sending commands to agents...".to_string();
             }
             KeyCode::Left | KeyCode::Char('h') => {
                 self.select_previous_host();
@@ -156,6 +159,15 @@ impl App {
     pub fn should_quit(&self) -> bool {
         self.should_quit
     }
+    
+    pub fn check_refresh_request(&mut self) -> bool {
+        if self.refresh_requested {
+            self.refresh_requested = false;
+            true
+        } else {
+            false
+        }
+    }
 
     #[allow(dead_code)]
     pub fn status_text(&self) -> &str {
@@ -256,6 +268,10 @@ impl App {
             self.zmq_subscription.clone(),
         ))
     }
+    
+    pub fn zmq_endpoints(&self) -> &[String] {
+        &self.zmq_endpoints
+    }
 
     pub fn handle_app_event(&mut self, event: AppEvent) {
         match event {
@@ -337,6 +353,10 @@ impl App {
 
                 self.status = format!("Fetch failed • host: {} • {}", host, error);
             }
+            AppEvent::RefreshRequested => {
+                // Handle refresh command - will be implemented in the main loop
+                self.status = "Refresh command sent to all agents".to_string();
+            }
         }
     }
 
@@ -641,5 +661,6 @@ pub enum AppEvent {
         error: String,
         timestamp: DateTime<Utc>,
     },
+    RefreshRequested,
     Shutdown,
 }