feat(dashboard): add real system metrics endpoint and fix UI dark-theme issues

- Add /api/system/metrics endpoint reading real data from /proc (CPU, memory, disk, network, load avg, uptime, connections)
- Replace hardcoded fake monitoring metrics with live API data
- Replace random chart data with real latency/error-rate/client-request charts from DB logs
- Fix light-mode colors leaking into dark theme (monitoring stream bg, settings tokens, warning card)
- Add 'models' to page title map, fix System Health card structure
- Move inline styles to CSS classes (monitoring-layout, monitoring-stream, token-item, warning-card)
- Prevent duplicate style injection in monitoring page

src/dashboard/mod.rs

@@ -92,6 +92,7 @@ pub fn router(state: AppState) -> Router {
         )
         .route("/api/providers/{name}/test", post(providers::handle_test_provider))
         .route("/api/system/health", get(system::handle_system_health))
+        .route("/api/system/metrics", get(system::handle_system_metrics))
         .route("/api/system/logs", get(system::handle_system_logs))
         .route("/api/system/backup", post(system::handle_system_backup))
         .route(