Remove all unused code and fix build warnings

- Remove unused struct fields: tier, config_name, last_collection_time
- Remove unused structs: PerformanceMetrics, PerfMonitor
- Remove unused methods: get_performance_metrics, get_collector_names, get_stats
- Remove unused utility functions and system helpers
- Remove unused config fields from CPU and Memory collectors
- Keep config fields that are actually used (DiskCollector, etc.)
- Remove unused proxy_pass_url variable and assignments
- Fix duplicate hostname variable declaration
- Achieve zero build warnings without functionality changes

dashboard/src/ui/mod.rs

@@ -140,9 +140,6 @@ impl TuiApp {
         
         // Get the current hostname (localhost) for auto-selection
         let localhost = gethostname::gethostname().to_string_lossy().to_string();
-
-        // Prioritize localhost if it becomes available, but respect user navigation
-        let localhost = gethostname::gethostname().to_string_lossy().to_string();
         if !self.available_hosts.is_empty() {
             if self.available_hosts.contains(&localhost) && !self.user_navigated_away {
                 // Localhost is available and user hasn't navigated away - switch to it