Update version to 0.1.21 and fix dashboard data caching

- Separate dashboard updates from email notifications for immediate status aggregation
- Add metric caching to MetricCollectionManager for instant dashboard updates
- Dashboard now receives cached data every 1 second instead of waiting for collection intervals
- Fix transmission to use cached metrics rather than triggering fresh collection
- Email notifications maintain separate 60-second batching interval
- Update configurable email notification aggregation interval

agent/src/config/mod.rs

@@ -143,6 +143,8 @@ pub struct NotificationConfig {
     pub from_email: String,
     pub to_email: String,
     pub rate_limit_minutes: u64,
+    /// Email notification batching interval in seconds (default: 60)
+    pub aggregation_interval_seconds: u64,
 }