Remove unused code and eliminate build warnings

Removed unused widget subscription system, cache utilities, error variants,
theme functions, and struct fields. Replaced subscription-based widgets
with direct metric filtering. Build now completes with zero warnings.

agent/src/collectors/error.rs

@@ -7,16 +7,4 @@ pub enum CollectorError {
     
     #[error("Failed to parse value '{value}': {error}")]
     Parse { value: String, error: String },
-    
-    #[error("System command failed: {command}: {error}")]
-    CommandFailed { command: String, error: String },
-    
-    #[error("Configuration error: {message}")]
-    Configuration { message: String },
-    
-    #[error("Metric calculation error: {message}")]
-    Calculation { message: String },
-    
-    #[error("Timeout error: operation took longer than {timeout_ms}ms")]
-    Timeout { timeout_ms: u64 },
 }