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/mod.rs

@@ -2,7 +2,6 @@ use async_trait::async_trait;
 use cm_dashboard_shared::Metric;
 use std::time::Duration;
 
-pub mod cached_collector;
 pub mod cpu;
 pub mod memory;
 pub mod disk;