Implement per-service disk usage monitoring

Replaced system-wide disk usage with accurate per-service tracking by scanning
service-specific directories. Services like sshd now correctly show minimal
disk usage instead of misleading system totals.

- Rename storage widget and add drive capacity/usage columns
- Move host display to main dashboard title for cleaner layout
- Replace separate alert displays with color-coded row highlighting
- Add per-service disk usage collection using du command
- Update services widget formatting to handle small disk values
- Restructure into workspace with dedicated agent and dashboard packages

shared/src/envelope.rs

@@ -17,3 +17,6 @@ pub struct MetricsEnvelope {
     #[serde(default)]
     pub metrics: Value,
 }
+
+// Alias for backward compatibility
+pub type MessageEnvelope = MetricsEnvelope;