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/config/defaults.rs

@@ -46,9 +46,6 @@ pub const DEFAULT_SMART_WEAR_CRITICAL: f32 = 90.0;
 // Backup configuration
 pub const DEFAULT_BACKUP_MAX_AGE_HOURS: u64 = 48;
 
-// Cache configuration
-pub const DEFAULT_CACHE_TTL_SECONDS: u64 = 30;
-pub const DEFAULT_CACHE_MAX_ENTRIES: usize = 10000;
 
 // Notification configuration (from legacy)
 pub const DEFAULT_SMTP_HOST: &str = "localhost";