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

agent/src/collectors/error.rs

@@ -0,0 +1,53 @@
+use thiserror::Error;
+
+#[derive(Debug, Error)]
+pub enum CollectorError {
+    #[error("Command execution failed: {command} - {message}")]
+    CommandFailed { command: String, message: String },
+
+    #[error("Permission denied: {message}")]
+    PermissionDenied { message: String },
+
+    #[error("Data parsing error: {message}")]
+    ParseError { message: String },
+
+    #[error("Timeout after {duration_ms}ms")]
+    Timeout { duration_ms: u64 },
+
+    #[error("IO error: {message}")]
+    IoError { message: String },
+
+    #[error("Configuration error: {message}")]
+    ConfigError { message: String },
+
+    #[error("Service not found: {service}")]
+    ServiceNotFound { service: String },
+
+    #[error("Device not found: {device}")]
+    DeviceNotFound { device: String },
+
+    #[error("External dependency error: {dependency} - {message}")]
+    ExternalDependency { dependency: String, message: String },
+}
+
+impl From<std::io::Error> for CollectorError {
+    fn from(err: std::io::Error) -> Self {
+        CollectorError::IoError {
+            message: err.to_string(),
+        }
+    }
+}
+
+impl From<serde_json::Error> for CollectorError {
+    fn from(err: serde_json::Error) -> Self {
+        CollectorError::ParseError {
+            message: err.to_string(),
+        }
+    }
+}
+
+impl From<tokio::time::error::Elapsed> for CollectorError {
+    fn from(_: tokio::time::error::Elapsed) -> Self {
+        CollectorError::Timeout { duration_ms: 0 }
+    }
+}