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

dashboard/Cargo.toml

@@ -18,3 +18,4 @@ tracing = "0.1"
 tracing-subscriber = { version = "0.3", features = ["fmt", "env-filter"] }
 tracing-appender = "0.2"
 zmq = "0.10"
+gethostname = "0.4"